Tritiated thymidine incorporation, a measure of DNA synthesis, was studied in circulating leukocytes from patients with widespread psoriasis who were being treated with photochemotherapy using oral 8-methoxypsoralen (8-MOP) and high-intensity, long-wave ultraviolet light (UVA). Seven of 13 psoriasis patients treated with photochemotherapy demonstrated a significant (p less than 0.05) reduction in leukocyte incorporation of tritiated thymidine immediately after UVA in comparison to incorporation before UVA. None of 10 control subjects treated with UVA alone demonstrated such reduction in leukocyte tritiated thymidine incorporation. Photochemotherapy thus affects circulating blood cells in some patients with psoriasis in addition to its therapeutic effect on epidermal cells. Further investigations are needed to determine the reasons for the differences in susceptibility to inhibition of leukocyte DNA synthesis among patients and the possible long-term consequences of such inhibition.