Ultraviolet-A (UV-A) light penetrates the epidermis, reaches the macrophages and circulating mononuclear cells within the dermis, and has immunoregulatory effects in humans. We examined the effect of UV-A irradiation on disease activity in 26 patients with rheumatoid arthritis (RA) and on immunologic function in these patients and in 11 normal subjects. Ten joules/cm2/day of total body UV-A irradiation, given 5 days each week for 3 weeks, resulted in significant improvement in the duration of morning stiffness, fatigue, joint tenderness, joint swelling, grip strength, patient assessment of disease activity, and physician assessment of disease activity. Platelet counts decreased significantly in the RA patients. Phytohemagglutinin (PHA)-stimulated lymphocyte production of interleukin-2 (IL-2) increased significantly in the combined RA and normal groups. These results suggest that UV-A light may be effective in the treatment of patients with RA, but elucidation of its precise role will require further study including double-blind trials.