The Mayo Clinic and its hospitals -- 14,000 staff members treating as many as 4,000 patients a day -- has become the nation's largest medical center to ban smoking.

"There are other hospitals and clinics with no-smoking rules, but they still have designated smoking areas," said a spokesman for the Rochester, Minn., medical center, which attracts patients from all over the world.

There are two exceptions to the Mayo rule: psychiatric and chemical (drug or alcohol) dependency patients, two already seriously stressed groups whose treatment might be hampered by telling them they may not smoke. Otherwise, the rule is complete for staff and patients. It took effect June 29.

The clinic gave staffers several months' warning. More than 80 percent were already nonsmokers. "There's a lot of empathy for the hardship this will cause for some people, fellow employes and patients," said Dr. Kenneth Berge, chairman of a task force overseeing the new policy. "But I think the greater good, the clear message we're sending as a health-care institution, will justify all the troubles . . . We're on the forefront of a movement that I think is going to sweep the country in the next few years."