In a prospective, double-blind comparison, we assessed the efficacy of transdermal clonidine with that of chlordiazepoxide in the treatment of moderately severe acute alcohol withdrawal syndrome. While having significant withdrawal symptoms, 50 hospitalized men were randomly assigned to receive either transdermal clonidine or chlordiazepoxide over a 4-day study period. Outcome was evaluated daily, medically and psychiatrically, using both objective and subjective measurements for dependent variables. No patient in either study group had seizures or progression to delirium tremens. The group receiving transdermal clonidine had a more significant response globally for the signs and symptoms of alcohol withdrawal, as measured by the Alcohol Withdrawal Assessment Scale. Also, clonidine more effectively lowered elevated systolic and diastolic blood pressure and heart rate. The core target symptom, anxiety, decreased significantly more in the patients receiving transdermal clonidine when measured by the Hamilton Anxiety Rating Scale and its subscale for somatic anxiety. Cognitive function responded equally in both study populations. Clonidine-treated patients reported less diarrhea, dizziness, headache and fatigue, and the chlordiazepoxide-treated patients reported less nausea and vomiting. We conclude that transdermal clonidine is effective treatment for the acute alcohol withdrawal syndrome.