Serum estradiol levels were compared in smoking and nonsmoking men in two separate samples. Sample I consisted of 41 young adult male volunteers ranging in age from 18 to 24 years. Twenty-three men smoked an average of 24.5 +/- 6.9 cigarettes daily. The duration of smoking averaged 5.2 +/- 2.2 years. Sample II consisted of 35 husbands who had been evaluated for infertility; they ranged in age from 19 to 49 years. Eighteen men smoked an average of 21.6 +/- 7.9 cigarettes daily. The duration of smoking averaged 11.5 +/- 4.5 years. Age, height, and weight did not differ significantly between smokers and nonsmokers within either group. Serum estradiol levels were significantly elevated in smokers compared with nonsmokers in both groups (p less than 0.001 and p less than 0.0001 in Samples I and II, respectively). No significant correlations were found between serum estradiol levels and the number of cigarettes smoked daily, or with the duration of smoking in either sample. The differences in serum estradiol levels between smokers and nonsmokers could not be attributed to the differences in marijuana and alcohol use that existed between the smokers and nonsmokers in each sample. The recent reports of elevated serum estradiol levels as a possible risk factor in coronary heart disease are discussed in view of the known relationship of cigarette smoking to coronary heart disease.