This investigation examined measurable personality differences between carefully selected samples of 40 heroin addicts and 40 nonaddict prisoners. Composite MMPI profiles were compared for statistical differences between groups, and individual MMPI profiles were classified using a system of differential diagnosis reported by Mechlin 1956. Results suggested that there are measurable personality differences between heroin addicts and nonaddict prisoners, especially with respect to the incidence and extent of sociopathy. The two samples did not differ from each other in terms of age, educational level, intellectual level, or chronicity of antisocial behavior as indicated by lime served in prison.