Computerised tomography (CT) was used to assess the possible effects of substance abuse on brain morphology. Polydrug abusers had significantly wider third ventricles than normal controls, with a positive correlation between age and ventricle:brain ratio (VBR). Assuming no effect of age, estimated quantity of substance abuse was not significantly related to ventricular and sulcal measures, except that alcohol consumption correlated positively with VBR and severity of cocaine use correlated negatively with sulcal width. When age of the subjects was partialled out, alcohol use showed a tendancy for association with VBR; however, severity of cocaine use did not remain a significant predictor of cortical sulcal width. The findings suggest that chronic use of alcohol, but not necessarily of other commonly abused substances, produces brain atrophy.