Previous studies from our laboratory have demonstrated that 25-hydroxyvitamin D3 (25OHD3) therapy is effective in raising the impaired intestinal calcium absorption (alpha) associated with postmenopausal osteoporosis. In the present study we have assessed the effects of long term 25-OHD3 therapy (50 micrograms/day; mean treatment period, 1.3 yr) in 12 women with postmenopausal osteoporosis (mean age, 62.5 yr). Our results indicate that there was a significant increase in alpha for the group during therapy. However, we found that the patients could be divided into 2 groups based upon their ability to raise alpha in response to 25OHD3 therapy. In those who responded (n = 7), alpha increased from 0.36 +/- 0.05 to 0.49 +/- 0.08 (+/- SD; P less than 0.005) while no significant change was observed for the nonresponders (0.44 +/- 0.03 to 0.48 +/- 0.07). During therapy, there were significant increases in serum 25OHD and 24,25-dihydroxyvitamin D [25,25-(OH)2D] for both groups. Serum 1,25-(OH)2D significantly increased in the responders (21 +/- 8 to 39 +/- 13 pg/ml; P less than 0.01), but not in nonresponders (25 +/- 11 to 28 +/- 8 pg/ml). Between-group comparisons for responders vs. nonresponders before therapy disclosed significant reductions in 24,25-(OH)2D (0.4 +/- 0.3 vs. 2.2 +/- 0.8 ng/ml; P less than 0.005) and alpha (0.36 +/- 0.05 vs. 0.44 +/- 0.03; P less than 0.01). During therapy, there were no significant differences in any parameter between the two groups, except for serum I,25-(OH)2D which was significantly higher in the responders (39 +/- 13 vs. 28 +/- 8 pg/ml; P less than 0.05). These data would suggest that in postmenopausal osteoporosis, the ability to raise alpha in response to 25OHD3 therapy is due in part to increases in serum 1,25-(OH)2D during therapy. This suggests that in some patients with menopausal osteoporosis, renal 25OHD3-1 alpha-hydroxylase may be impaired.