The effects of postmenopausal hormone replacement therapy (HRT) and vitamin D3 on vitamin D metabolites (25OHD and 1,25(OH)2D) were studied in a population-based prospective 1-year study. The serum concentrations of intact parathyroid hormone (PTH), calcium, and phosphate were also studied. A total of 72 women were randomized into four treatment groups: HRT group (sequential combination of 2 mg estradiol valerate and 1 mg cyproterone acetate), Vit D3 group (vitamin D3 300 IU/day + calcium lactate 500 mg/day), HRT + Vit D3 group (both above) and placebo group (calcium lactate 500 mg/day). Serum samples were taken in March-April, when vitamin D formation from sunlight in Finland is minimal after the dark winter. Serum concentrations of 25OHD increased in the Vit D3 group (33.5%, P < 0. 001) and in the HRT + Vit D3 group (38.2%, P < 0.001) but had not changed significantly in the HRT and placebo groups at the 1-year follow-up examination. Serum concentrations of calcitriol (1, 25(OH)2D) increased, however, only in the HRT group (23.7%, P < 0. 05), and remained unchanged in other groups. Serum concentrations of PTH decreased by 23.2% (P < 0.05) in the placebo group, but did not change significantly in the other three groups. The concentrations of serum calcium increased in the nonhormone groups (P < 0.001), whereas serum phosphate concentrations decreased in the hormone groups (P < 0.05 and 0.001). Our results confirm the positive effect of 1 year of HRT on serum calcitriol. Vitamin D3 supplementation increased 25OHD concentrations, but did not affect calcitriol concentrations even though the initial levels were low. Interestingly, the combination of HRT and vitamin D3 did not increase serum calcitriol concentrations as much as HRT alone.