The aim of this study was to describe the effect of systemic alphacalcidol (1 OH vitamin D3) treatment on clinical and immunological parameters in patients with psoriatic arthropathy. Among the 19 patients investigated, 10 were treated with 0.25 microg oral alphacalcidol twice daily for 6 months, while 9 other patients served as controls. In the peripheral blood of the treated group but not in the controls, a statistically significant decrease was observed in the percentage of CD3/CD69-positive activated and CD8-positive interferon-gamma-producing T cells and in the serum level of interferon-gamma during the first 3 months and also in the clinical activity of the disease during the whole 6-month follow-up period. Our results show that systemic alphacalcidol treatment has an immunomodulatory effect on patients with psoriatic arthropathy. This effect is manifested by a short-term temporary decrease in type 1 immune responses and a continuous decrease in disease activity.