Sometimes the finest television is also the hardest to watch. Tonight's "Frontline" documentary on PBS is an achingly intimate study of a Midwestern family grappling with a life-and-death decision -- and it raises agonizing questions about what life and death have become.

"Let My Daughter Die," at 9 on Channel 26 and other public TV stations, is also a story still without an ending. Joe and Joyce Cruzan of Mount Vernon, Mo., are seeking through legal channels the right to disconnect life support systems now keeping their 30-year-old daughter Nancy alive.

Nancy lies in what doctors call a "persistent vegetative state" at the Missouri Rehabilitation Center, the result of brain damage she suffered when her car went out of control on an icy night in January 1983. She was technically dead for 20 minutes; when modern science brought her back to life, it restored basic bodily functions but not, it appears, thinking, feeling, believing.

The $130,000 in medical expenses required to keep Nancy alive each year are borne by the state, but the parents want the nutrition tube that feeds her removed so that she can escape her vegetative existence and so that they can escape what the narrator calls "the emotional limbo of having a daughter between life and death."

Nancy Cruzan does not require a respirator; she is breathing on her own. Her parents talk of death with dignity, but with the feeding tube removed her death would be by starvation, over a period estimated at 10 to 15 days.

Nancy's family and friends granted the makers of the documentary extraordinary access to their lives at almost unbearably delicate moments -- the meeting of a seminar on stress management at which the parents announce their decision, Nancy's sad 30th-birthday party, held at her hospital bed while she stares blankly ahead.

In the wrong hands, material like this could have become demeaningly maudlin; one can imagine what one of the TV tabloids like "A Current Affair" or "Geraldo" would do with it. But the story is in the right hands, those of producer Elizabeth Arledge and her crew, who made the hour straightforward and dispassionate yet not coldly clinical.

Arledge, the daughter of ABC News and Sports President Roone Arledge, shot the program on vivid videotape and captured the strengths and the anguish of the Cruzans and their community. Nancy's sister Christy is especially articulate and open; her daughters worship their aunt, and when one of them tries to recall casually a happy shared moment from the past, the daughter simply shatters in sudden tears. She is unable to go on. The camera withdraws.

What the program communicates is the whole new set of values and perspectives that even the more mundane medical miracles bring with them. "Sharing pain" may be an '80s cliche', but the Cruzans' pain is real, and you do share it. You all but live through it with them.

"Let My Daughter Die" touches on the deepest issues of all.