The loss of bone which starts at the menopause is self-limiting (exponential) and possibly mainly trabecular. It merges into an age-related linear loss of bone which is probably mainly cortical. The menopause is associated with a rise in obligatory urinary calcium loss resulting from an increase in the filtered load of calcium which may be due to the complexed fraction. The dependence of the urinary hydroxyproline on the urinary calcium and sodium suggests that the bone resorption is a response to calcium losses rather than a primary event. In osteoporotic women, there is a further increase in filtered load of calcium and obligatory calcium loss, frequently coupled with malabsorption of calcium. Urinary hydroxyproline can be suppressed by calcium administration in those with normal absorption and by calcitriol in those with calcium malabsorption. It is known that calcium deficiency causes osteoporosis in experimental animals, but there is controversy about the role of calcium deficiency in the pathogenesis of human osteoporosis. Calcium supplementation inhibits cortical bone loss in postmenopausal women but there is some doubt as to whether it can inhibit trabecular bone loss in women close to the menopause. This may be partly a matter of dose, formulation and time of administration.