One hundred eleven mildly to moderately insomniac patients participated in a double-blind, placebo-controlled study to establish the efficacy of diphenhydramine as an over-the-counter (OTC) sleep aid. A two-week crossover design was employed in which all patients received both diphenhydramine and placebo for one week each. The daily diphenhydramine dose was 50 mg at bedtime. Results obtained indicate that diphenhydramine improved various sleep parameters, including sleep latency, to a significantly higher degree than did placebo. In addition, patients on diphenhydramine reported feeling more restful the following morning and patients preferred the diphenhydramine drug to placebo despite experiencing more side effects. This study thus supports the use of 50 mg diphenhydramine as an OTC sleep aid in the treatment of temporary mild to moderate insomnia.