Objective: To study the calcium homeostasis in healthy, calcium and vitamin D replete early postmenopausal women during oral supplementation with calcium and vitamin D₃. Design: A prospective, placebo-controlled, randomised, double-single-blind, 3-week study. Setting: Outpatient clinic at Copenhagen University Hospital, Denmark. Subjects: In all, 17 started, one was excluded. Totally, 16 healthy women, 45-61 y of age (mean 57.3 y) who were at least 4 y after menopause (mean 6.7 y) completed. Interventions: All underwent three consecutive 7-day study periods. Each began with 4 days of normal diet followed by 3 days treatment of either C: one tablet of 1.250 mg calcium carbonate (ie 500 mg Ca²⁺ per tablet) twice daily (breakfast and dinner), or CD₃: as in C but plus 400 IU vitamin D₃ b.i.d., or P (only) placebo tablets b.i.d. Results: At baseline plasma 25-hydroxycholecalciferol was normal (66±22 nmol/l) and the calcium intake without supplements 850 mg/day. In group C, the 24-h urinary calcium increased by 35% (6.9±2.0 mmol), vs the placebo group P (5.1±1.6 mmol) (P<0.05). Addition of 800 IU vitamin D₃ daily (CD₃) did not increase calcium excretion further (6.6±2.1 mmol) but decreased plasma 1,25-(OH ₂-vitamin D₃ by 21% (P<0.05). Conclusions: In this carefully controlled study calcium plus vitamin D₃ supplements only had minor influences of uncertain significance on the calcium balance in healthy, calcium and vitamin D sufficient early postmenopausal women.