For 18 years, a patient with idiopathic hypoparathyroidism has experienced stiffness or tetany chiefly at the time of menses. A significant decline (0.2 mmol/L) in her serum total and ionized calcium values was observed after the withdrawal of oral contraceptive and after spontaneous onset of menses. No increase in urinary calcium excretion or decline in serum magnesium or calcitriol occurred to explain the fall in serum calcium. In two hypoparathyroid women without the history of menses-associated tetany, serum calcium remained stable after withdrawal of oral contraceptive or conjugated estrogen. Serum calcitriol levels unexpectedly increased in these controls, possibly helping sustain serum calcium. The author concludes the following: (1) some hypoparathyroid women show a significant and symptomatic decline in serum calcium at the time of menses; (2) serum calcium values obtained at the time of menses may not be a reliable guide to adjusting medication dosage for hypoparathyroidism; and (3) further investigation will be needed to determine the mechanism of the decline in serum calcium values and whether the effect of estrogens on serum calcitriol might be altered in the absence of the parathyroid glands.