Photosensitivity investigations have been carried out using an irradiation monochromator on 31 patients taking one of seven different nonsteroidal anti-inflammatory agents for the treatment of rheumatoid and osteoarthritis. Six patients, who were taking either piroxicam, naproxen or tiaprofenic acid, experienced adverse immediate reactions of erythema and flaring, together with an urticarial response in four of these six patients. No phototoxic response was observed in patients taking either indomethacin, ketoprofen, benorylate or ibuprofen, although firm conclusions about the non-phototoxic nature of these four drugs cannot be drawn from this pilot study because of the small numbers of patients investigated.