1. We have examined the relation between the rate of cortical bone loss and other measured variables in 108 women, untreated and on various therapies, observed for nearly 300 patient years. The majority of the subjects were post-menopausal women with various degrees of simple osteoporosis, but a few cases of hyperparathyroidism and other calcium disorders were included. 2. Calcium balance, bone resorption rate, urinary hydroxyproline and plasma alkaline phosphatase were shown to be related to the rate of bone loss; patients with the more negative balance or the higher values of the other variables had significantly higher rates of loss. The rate of bone loss was independent of the rate of bone formation.

1. Plasma alkaline phosphatase activity and urine hydroxyproline excretion were measured in age-matched premenopausal and postmenopausal women. 2. Both measurements were found to be significantly higher in postmenopausal women. 3. It is proposed that the rise in the plasma alkaline phosphatase, like that in the hydroxyproline excretion, may reflect the onset of bone loss that occurs after the menopause.