We compared changes over 24 h in 15 postmenopausal normal women (mean [+/- SD] age, 64 +/- 7 yr) with those in 15 postmenopausal women with type I osteoporosis and vertebral fractures (mean age, 64 +/- 5 yr). The serum osteocalcin concentration, a sensitive index of bone formation, increased by about 5% at night in both groups. Urinary deoxypyridinoline excretion, a sensitive index of bone resorption, increased by 48% at night (P less than 0.01) in the normal women, whereas in the osteoporotic women it was 62% higher overall (P less than 0.05), and the increase persisted into the morning. At night, urinary fractional excretion of calcium decreased by 20% (P less than 0.001) in the normal women, but was unchanged in the osteoporotic women; this circadian pattern differed between groups (P less than 0.05). The serum ionized calcium concentration did not change at night in either group. There was a trend (P = 0.07) for blunting of the nocturnal increase in the serum intact PTH level in osteoporotic women. Thus, the nocturnal serum ionized calcium level is maintained by decreased urinary calcium excretion and increased bone resorption in postmenopausal normal women, but almost entirely by increased resorption in postmenopausal osteoporotic women. This greater dependence on bone resorption during the nocturnal fast may account in part for the greater bone loss in osteoporotic women.