1. We studied the mechanism of action of the anabolic steroid, stanozolol, in 23 patients with osteoporosis, using a combination of biochemical and histomorphometric techniques.

2. Treatment with stanozolol (5 mg/day) for 1 year was associated with a marked and significant decrease in the fasting urinary excretion of calcium (P < 0.01) but not with changes in the serum concentrations of calcium and phosphate, the serum activity of alkaline phosphatase, the renal tubular reabsorption of calcium or the urinary excretion of hydroxyproline.

3. Histological examination of trabecular bone showed an increase in bone turnover and the bone formation rate increased twofold (P < 0.02). There were no significant changes in bone volume or wall thickness after treatment. Tetracycline labelling was used to discriminate bone structural units completed before and during treatment. Measurements of the wall thickness of those bone structural units formed during treatment showed no significant change.

4. Measurements made on the endocortical surface also showed an increase in bone turnover, but in contrast to trabecular bone, the bone structural units formed at endocortical sites during treatment had a significantly greater wall thickness than those formed before treatment (P < 0.05).

5. We conclude that stanozolol increases the turnover of trabecular bone and increases the endocortical apposition of bone.

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