The usefulness of the antihistaminic agent diphenhydramine hydrochloride was evaluated using a double-blind procedure at sleeping doses of 12.5, 25, and 50 mg in 144 psychiatric patients with insomnia. The general condition of the patients with insomnia was at least "slightly improved" in 62.5% (12.5-mg group), 60% (25-mg group), and 67.4% (50-mg group) after treatment with the test drug for 2 weeks. Side effects were observed in a total of 11 patients (7.6%) but were not severe. No symptoms suggestive of drug dependence were evident. Global improvement was not influenced by patient background factors except for the presence or absence of previous treatment for insomnia. The hypnotic effect of diphenhydramine hydrochloride was significantly greater in patients who had not been treated previously. A dose-dependent increase in the hypnotic effect was also seen in patients who had not received any previous treatment. Diphenhydramine hydrochloride thus appears to be effective in the treatment of insomnia, but the appropriate dosage will depend on previous medical treatment of insomnia.