Oral azathioprine was compared with placebo in a double-blind controlled trial of therapy in chronic actinic dermatitis (CAD), a rare eczematous photodermatosis. Eighteen severely affected patients were randomly allocated to azathioprine 50 mg t.d.s. or placebo over a 2-year period. Severity of itch and rash were assessed weekly by each patient on a visual analogue scale and overall clinical status monthly by a medical observer. Monitoring of patient ultraviolet radiation (UVR) exposure was undertaken throughout treatment by polysulphone film lapel-badge dosimetry. Five of 8 patients treated with azathioprine but none of 10 placebo patients achieved remission within 6 months. One patient could not tolerate treatment because of gastrointestinal effects. No haematological or hepatic abnormality was noted. The marked improvement in clinical status of actively treated patients (P less than 0.02, Fisher's exact test), led to early termination of the trial. Oral azathioprine therapy is an effective and usually well tolerated treatment in chronic actinic dermatitis.