The efficacy of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and the new analogue calcipotriene (MC 903) in the treatment of psoriasis was investigated. Eight patients with psoriasis were enrolled in a pilot study with systemically administered vitamin D3 and were given 1,25(OH)2D3 (Rocaltrol; Hoffmann-La Roche) in dosages from 0.5 microgram to 2 micrograms/day for a 6-month trial. In one patient, the psoriatic plaques resolved within 2 months after treatment (0.5 microgram/day) was initiated. Moderate improvement was noted in one other patient (1 microgram/day). The serum level of 1,25(OH)2D before treatment was not less than the normal range of the adult population. Side effects of systemically administered 1,25(OH)2D3 included a dose-dependent increase in the 24-hour urinary calcium excretion and a decrease in the total number of platelets. Seven patients with symmetric plaque-type psoriasis were treated topically with 2 micrograms/g of 1,25(OH)2D3 in petrolatum. During 3 months of follow-up, mild improvement was noted in three patients. Five patients in the calcipotriene study were part of a nationwide double-blind placebo-controlled trial by Bristol-Myers Squibb. Moderate to marked improvement was noted in the two patients who received 50 micrograms/g of calcipotriene topically. The three patients who received placebo showed no response. We conclude that a subset of patients with psoriasis responds well to 1,25(OH)2D3. Calcipotriene is efficacious and an excellent alternative to topically applied corticosteroids.