Serum levels of vitamin D metabolites have been studied in 68 patients 1-131 months (mean 23 +/- 23) after kidney transplantation. They were separated into two groups according to the duration of the graft. Patients with a graft duration longer than 6 months had a mean GFR of 73 +/- 36 ml/min. Their serum levels of 25-hydroxyvitamin D (25-OHD) were normal (15.9 +/- 8.4 ng/ml), while serum levels of 1,25-dihydroxyvitamin D (1,25-OHD) were reduced (28 +/- 11 pg/ml). A highly significant correlation between GFR and 1,25-OHD was observed (r = 0.58; p less than 0.001). Most patients with normal renal function had levels of 1,25-OHD in the normal range, but they frequently remained slightly lower than those of healthy controls (33 +/- 9 vs. 38 +/- 10 pg/ml; p = 0.05), albeit the presence of elevated PTH and/or hypophosphatemia. In some patients, a low supply of 25-OHD could impair the normalization of 1,25-OHD levels. No clear differences in vitamin D metabolites were observed between patients transplanted 1-6 months ago and those with a longer duration of the graft. Nevertheless, serum 25-OHD showed a tendency to increase with time. These results indicate that, although 1,25-OHD levels may not be fully normalized, successful kidney transplantation restores the ability to effectively metabolize vitamin D.