There is a belief that persons diagnosed with antisocial personality disorder (APD) do not respond well to treatment, but the existing research has not supported this hypothesis. This study examined the relationship of APD to therapeutic community (TC) treatment outcomes. A total of 275 men and women were randomly assigned to two TCs. It was hypothesized that clients diagnosed with APD via the Millon Clinical Multiaxial Inventory (MCMI-II) would have poorer treatment outcomes than those with no APD. The MCMI-II was used to diagnose APD because of its focus in underlying pathological personality traits, as opposed to strict behavioral criteria as used in the Diagnostic and Statistical Manual of Mental Disorders classifications for APD; this hypothesis was not supported. Logistic regression analyses indicated that an MCMI-II diagnosis of APD was unrelated to treatment outcomes. Treatment completion was the most important factor in reducing recent drug use and post-discharge arrests. The results indicate that persons diagnosed with APD, with histories of substantial drug abuse and criminality, can benefit from TC treatment with aftercare in the community or at the very least, do as well as those with no APD. In light of the high prevalence rates of APD in substance-abusing populations, future research should continue to explore the many issues surrounding the diagnosis of APD, as well as its relationship to treatment outcomes.