The present study was conducted to test the hypothesis that individuals with substance abuse disorder exhibit structural deficits in the prefrontal cortex. Volumes of the prefrontal lobe in subjects with histories of polysubstance abuse (n = 25) were measured and compared with those in normal volunteers (n = 14), using high-resolution volumetric magnetic resonance imaging (MRI). The research participants were men, 22 to 41 years of age. Polysubstance abusers were abstinent from drugs of abuse (except nicotine) for at least 15 days before MRI scanning. The total volumes of the prefrontal lobe (left and right hemispheres) were significantly smaller in the substance abuse group than in the control group. When the prefrontal lobe was segmented for gray and white matter, the deficit in the substance abusers was seen as significantly smaller volumes of gray but not of white matter. These results indicate that hypoplasia and/or atrophy in the prefrontal cortex accompany substance abuse and suggest that structural deficits in the prefrontal cortex may play an essential role in the neuropathological basis of functional impairments in substance abuse disorder, as demonstrated by functional brain imaging and cognitive studies.