Except for alcohol abuse, little is known about the familial aggregation for substance abuse. Here we report twin resemblance for non-alcohol substance use in the Washington University Twin Series, wherein probands were identified by consecutive admission to psychiatric facilities in the St. Louis area. A 5-point substance abuse scale was constructed with values anchored by never used drugs (1) to drug dependence (5). Year of birth was the most powerful predictor of drug use--younger twins scored far higher than older twins. Either heritability or common environment had to be included in the regression model to avoid a significant drop in explained variance, but which was more important could not be resolved. The correlation for identical twins exceeded that for fraternal twins, suggesting the possibility of a heritable factor.