Fifty patients with chronic idiopathic urticaria were studied to compare responses to treatment with doxepin (10 mg three times a day) and with diphenhydramine (25 mg three times a day). All patients had an evaluation that failed to disclose a cause for their disease. Therapeutic response was assessed according to the suppression of symptoms and symptom diary scores of daily itching and frequency, number, size, and duration of hives. Total clearing of the pruritus and urticarial lesions occurred in 43% of the patients while receiving doxepin and in only 5% while receiving diphenhydramine (p less than 0.001). Partial or total control of the pruritus and hives was noted in 74% of the patients receiving doxepin and in only 10% of those receiving diphenhydramine (p less than 0.001). Doxepin induced markedly less sedation (22%) than diphenhydramine (46%) (p less than 0.05). Dermatopathologic categories included (1) urticaria simplex, (2) lymphocytic urticaria, and (3) leukocytoclastic urticaria. Patients with urticaria simplex had a more favorable response to doxepin than the two other groups.