This paper examines the heritability of cigarette smoking and alcohol consumption in 360 adult, male twin pair participants (176 monozygotic and 184 dizygotic pairs) in the second exam of the National Heart, Lung, and Blood Institute's Twin Study. Heritability estimates for smoking and alcohol use were calculated both before and after adjustment for shared variance between these behaviors and other characteristics, including coffee consumption, contact between twins, and two psychological traits: anger and activity. The purpose of the analysis was to determine the impact of adjustment for covariates on heritability estimates of smoking and alcohol use. Before adjustment, heritability of both smoking and alcohol use was highly significant and accounted for 52% and 60% of the variance, respectively. After adjustment for covariates, the heritability of smoking remained at 52% while that for alcohol use decreased to 43%. The fact that these estimates remained significant after adjustment for covariates leads to increased confidence about the role of genetics in both smoking and alcohol consumption.