IT'S DECEMBER 2006, and I'm high up on a rock wall in Zion National Park, looking down at Jeff Lowe as he fiddles gear into a tiny crack that splits an otherwise flawless pane of tan, vertical sandstone. It's not pretty—his movements are slow, weak, and gummy. Lowe, 57, is one of the greatest climbers who's ever touched rock or ice, but since 1999 he's been afflicted with multiple sclerosis, a disease of the nervous system that can lead to terrible physical disabilities. For Lowe, the worst damage has been to his legs, which are dragging behind him like dead fish. This morning, near the base of the route, he could barely shuffle across a parking lot, even with the aid of two canes. The trail to the wall's base, a few hundred feet uphill, required 40 minutes of teetering, stumbling, and crawling.

Zion is an old haunt for Lowe. In the 1970s, he pioneered some classic climbs here—notching several of his roughly 1,000 first ascents—but today he's on a novice route, a 5.9 A1 called Touchstone Wall. What's more, he's direct-aid climbing, umbilicaled by nylon slings and engineering upward like a telephone repairman. During his prime, Lowe could have climbed these eight pitches in a few hours. Instead, he's still struggling to cover his first 30 feet.

Lowe clips into an aluminum chock he's wedged in the crack, then bends down and yards his right foot into an aider step with both hands. He repeats the process and sets a similar, thumbnail-size stopper a foot and a half higher. He squints through gold wire-rimmed glasses at the V-slot above. The desert sun is slowly reddening Lowe's pale skin while a breeze tousles his thinning platinum hair. He moves higher, pulling on the stopper, dragging a numb leg upward.

Suddenly, the stopper explodes from the grainy sandstone. Lowe's body hangs in midair before beginning its downward plunge. The rope tightens but then springs free as a second, lower piece is snatched from the crack, sending Lowe, arms outstretched, farther down the cliff, a hurtling bag of flesh and bone.

The rope finally catches, stopping Lowe with a bounce. He hangs there a moment, cocked sideways, before pulling himself upright. He looks up at the crack, thinking about the half-hour it will take to regain his high point. "Damn," he says, "now I have to do that all over again."

Unhurried and deliberate, Lowe straightens out his gear. He gropes his deadened quads and knees as if to manually check for injuries. Then, as he's done countless times over a legendary career spanning five decades and four continents, he sucks it up and goes—struggling, sweating, finessing some tricky gear past the crux. He spends several hours doing this, finally completing a 130-foot pitch.

That night, inside a hotel room in nearby Springdale, Utah, I draw Lowe a bath, testing the water to make sure it's not scalding, because he might get burned without feeling it. I put a glass of Scotch on the tub's rim. Lowe shuffles in, naked and paunchy. He eases into the water, hands grasping the support rails. "Thanks, Pete," he says with a laugh. "I feel like I've done an entire big wall instead of a single pitch."

Later, after his third Scotch, he says, "It's over—the fun is out of it, and that stuff up there wasn't even what I would call climbing." He sighs. "It confirmed what I already knew. I'll never climb again."