1. The time-course of the age-related decline in specific muscle force (maximum voluntary force per cross-sectional area) in men and women was determined by measuring the maximum voluntary force and cross-sectional area of the adductor pollicis muscle in 273 subjects aged 17-90 years (176 men, 30 premenopausal women and 67 peri- or post-menopausal women who were not receiving hormone replacement therapy). 2. To determine whether the loss of specific muscle force is hormone-dependent in women, we studied a further 25 women, aged 42-72 years, who were receiving hormone replacement therapy. 3. There was no significant difference in specific force between young men and pre-menopausal women. Around the time of the menopause there was a dramatic decline in specific force in women which was prevented by the use of hormone replacement therapy. In men the weakness started later (around the age of 60 years) and the decline in specific force was more gradual, reaching the level seen in post-menopausal women after the age of 75 years. 4. The protective effect of hormone replacement therapy on muscle strength is likely to be an important contributory factor to its proven action in preventing osteoporotic fractures. The dramatic peri-menopausal decline in muscle strength is a likely explanation for the known increases in falls and Colles' fractures around the time of the menopause.