1. The changes in serum calcium and phosphorus which occurred in forty-one patients with post-menopausal osteoporosis during treatment with stilboestrol were examined.

2. There was a fall of approximately 15 per cent in mean serum phosphorus and of approximately 3 per cent in mean serum calcium.

3. The fall in serum calcium is considered to furnish some evidence against the theory that the primary action of stilboestrol is a reduction in calcium excretion.