Seventy-one heavy smokers who had failed in previous attempts to stop smoking participated in a randomized clinical trial to test the efficacy of clonidine as an aid in smoking cessation. The success rate in clonidine-treated subjects (verified by serum cotinine concentration) was more than twice that in the placebo-treated subjects. When the data were stratified by gender, a strong effect present in women was not apparent in men. After six months, cessation rates remained significantly higher among smokers treated with clonidine than those receiving placebo. The data also revealed an unexpectedly high prevalence (61%) of a history of major depression in this sample and a significant negative effect of such a history on cessation regardless of treatment. These findings, highly suggestive of an important role of clonidine in smoking cessation, warrant further studies to establish the long-term (greater than or equal to 12 months) efficacy of this drug and to replicate the association between nicotine dependence and depression.