Estrogen deficiency might increase responsiveness of bone to circulating endogenous parathormone. To explore a possible relationship between parathormone action on bone and estrogens we studied the activity of the bone isoenzyme of serum alkaline phosphatase and the urinary excretion of hydroxyproline in 16 premenopausal and 24 postmenopausal women with primary hyperparathyroidism with hyperparathyroid osteodystrophy. The postmenopausal women with primary hyperparathyroidism had the B-ALP 4.30 +/- 0.54 mukat/l, the urinary hydroxyproline excretion 205.2 +/- 22.2 mmol/mol creatinine and urinary calcium excretion 8.9 +/- 0.5 mmol/24 hours, significantly increased in comparison with the group of women with menstrual cycle and primary hyperparathyroidism who had B-ALP 2.12 +/- 0.43 mukat/l, the urinary hydroxyproline excretion 119.0 +/- 14.9 mmol/mol creatinine and urinary calcium excretion 7.7 +/- 0.4 mmol/24 hours. Evidence supporting that estrogen deficiency might increase responsiveness of bone to circulating endogenous parathormone was provided by the demonstration that postmenopausal women with primary hyperparathyroidism had increased bone turnover assessed by urinary hydroxyproline excretion and bone isoenzyme of alkaline phosphatase in comparison with the group of premenopausal women with primary hyperparathyroidism.