“Thank God we can’t tell the future. We’d never get out of bed.”

Getting out of bed comes with an additional set of pitfalls at the Weston household, the setting of Tracy Letts’s savagely brilliant “August: Osage County.” This is in part because Violet Weston, the pill-popping, cancer-ravaged matriarch who treats her family like so many ducks in an emotional shooting gallery, has papered over the windows of her rambling Oklahoma home. (“Try not to differentiate between night and day,” Violet’s husband counsels the new housekeeper right before heading off on a bender.) But whether the Westons’s long days are journeying into nights or vice versa, the future has been all but predetermined by a ruinous past.

Mr. Letts, a fixture on the Chicago theater scene, is best known around these parts for two off-Broadway genre exercises, the gruesome trailer-trash noir “Killer Joe” and the supremely creepy sci-fi drama “Bug.” As juicy as those two plays were, however, they offered no inkling of the emotional depths on display in “Osage.” Packed with unforgettable characters and dozens of quotable lines, it is as harrowing a new work as Broadway has offered in years and the funniest in even longer.

It is also, not counting two- and three-part extravaganzas such as “Angels in America” and “The Coast of Utopia,” the longest new drama to reach Broadway in recent memory. Eugene O’Neill is a clear influence (along with Sam Shepard, Lillian Hellman, Edward Albee, and at least a half-dozen other writers), and Mr. Letts has revisited O’Neill’s leisurely tempos, as well as his subject matter. But to paraphrase Roger Ebert, no good play is too long and no bad play is short enough. And “August: Osage County,” which features a peerless cast and crew from Chicago’s Steppenwolf Theatre, is a very, very good play.

Director Anna D. Shapiro, who chronicled a comparably toxic family in last year’s “The Pain and the Itch” (another Chicago import), has pieced together every component beautifully, anchored by Todd Rosenthal’s suffocatingly cozy three-story set. And she displays a fantastic gift for manipulating the wide expanse of the Imperial Theatre, traditionally the province of large-scale musicals. As many as six individual tableaux take place throughout the house at once, but Ms. Shapiro makes it clear which fraught interaction we should be watching at any given point.

How we should be feeling, however, is another matter. The wounds visited on the three Weston daughters both by Violet and by their father, a hard-drinking cowboy poet of some repute named Beverly Weston (Dennis Letts, the playwright’s father), extend in all directions, creating a visceral blend of co-dependence and guarded hostility between Violet and the three emotionally crippled girls. Most of “Osage” unspools in the days and weeks after Beverly has abandoned the “cruel covenant” of his marriage and headed off on that bender, possibly for good. It is reasonable to assume that the loquacious, intensely narcissistic Beverly did his share of the talking when he was around; now that he’s gone, the entire family finds itself jockeying for supremacy while simultaneously itching to escape.

Barbara (Amy Morton), who walked away from a promising career as a writer to follow her husband, Bill (Jeff Perry), is the most willing to confront the malevolent Violet (Deanna Dunagan, in the performance of a lifetime), who uses her pill-induced fog to justify her hateful actions and slumps into self-pitying tears if challenged. Barbara was Daddy’s favorite, according to Violet; this comes as a surprise to middle sister Ivy (Sally Murphy), the only one to stay in the area and tend to her increasingly shambolic parents. And Karen (Mariann Mayberry), the baby of the family, talks about turning a new leaf with the intensity of a woman who has seen some terrible times; her new fiancé, a thrice-divorced back-slapper named Steve (Brian Kerwin), shows little promise of reversing Karen’s luck.

Also in attendance: Jean (Madeleine Martin), Barbara and Bill’s precocious 14-year-old daughter; Mattie Fay (Rondi Reed), Violet’s busybody sister; Mattie Fay’s husband and son, Charles and Little Charlie (Francis Guinan and Ian Barford), and Johnna (Kimberly Guerrero), the eminently capable American Indian housekeeper. These 11 characters splinter off, circle one another warily, and regroup before gathering for what may be the squirmiest, most shockingly funny dinner scene since Titus Andronicus served Tamora her own children.

Even as the play enters its fourth hour and these relatively minor characters (including the local sheriff, played by Troy West) each get their moment in the spotlight, Mr. Letts and Ms. Shapiro see to it that these brief vignettes never feel forced or obligatory. In fact, they have the opposite effect: A devastating family portrait emerges as even the most tangential relatives grapple with the family’s toxic aftershocks, proclaiming their refusal to dwell on the past even as they find themselves sucked deep into it. “‘Greatest Generation,’ my ass,” Barbara grumbles about her parents. “What makes them so great anyway? Because they were poor and hated Nazis?”

The few cavils about this extraordinary work come near the end, but they have less to do with fatigue and more to do with a few small narrative missteps. In depicting the younger generation as it puts its own stamp on the Weston pathologies, Mr. Letts pushes the symmetry slightly beyond what is necessary. As the family dissipates, so does the dramatic tension. And while the plot evolves plausibly on first viewing, the litany of eyebrow-raising developments (addiction, divorce, incest, pedophilia, suicide) threatens to turn Pawhuska, Okla., into a Great Plains version of Peyton Place upon reflection.

But such reflection also unearths line after line of intensely satisfying comedy, interspersed with remarkable evocations on the cruelties and (occasional) kindnesses of family life. It also yields the near-miraculous performances by Ms. Dunagan and Ms. Morton, who offer an emotional one-two punch unlikely to be bested anytime soon. In almost any other play, their work as a mother and daughter who are more alike than either cares to admit would stand out all the more. Surrounded by the overarching excellence of “August: Osage County,” they’re just two cantankerous cogs in a well-oiled yet gloriously squeaky machine.

Until March 9 (245 W. 45th St., between Broadway and Eighth Avenue, 212-239-6200).