A considerable increase in muscle strength and bone mass can be achieved in young adults through athletic exercise programs. We studied a less demanding nonloading exercise program for the back extensor muscles in postmenopausal women who were not on estrogen therapy. We randomly assigned 65 healthy Caucasian women without evidence of or risk factors for osteoporosis into an exercise group and a control group. The strength of the back extensor muscles and bone mineral density of the lumbar spine were measured at baseline and every 6 months for 2 years. In addition, a physical activity score was determined. Compliance was assessed by regular interviews and review of diaries. During the 2-year study, the mean rates of bone loss in the two groups were not statistically different. The strength of the back extensor muscles increased in both groups but significantly more (P = 0.002) in the exercise group. We conclude that postmenopausal bone loss is unaffected by a modest exercise program despite an increase in muscle strength. Nonloading muscle exercise may be ineffective in retarding vertebral bone loss in ambulatory, healthy postmenopausal women.