A double-blind, crossover study was carried out in 31 patients with rheumatoid arthritis or osteoarthritis who suffered from insomnia which was considered to be caused primarily by their disease. Patients received 7-day courses of 400 mg chlormezanone, 200 mg chlormezanone and placebo in a pre-determined random order. Patients rated chlormezanone significantly (p less than 0.025) more effective than placebo in overcoming sleep disturbance and preferred the 400 mg dose. There was also a trend towards better quality of sleep with chlormezanone, although this did not attain statistical significance in this relatively small study. Daytime alertness was similar for both active and placebo treatment periods. Chlormezanone, therefore, would seem to be a useful addition to antirheumatic therapy when there is related insomnia.