Studies of pregnant women receiving methadone maintenance have tended to focus on teratogenic, prenatal, and neonatal issues. We are not aware of any controlled studies comparing pregnant to non-pregnant heroin-addicted women in methadone treatment. This article presents findings from a study examining treatment outcome between pregnant and non-pregnant participants in a metropolitan methadone-maintenance program. Participants were 51 pregnant women and 51 non-pregnant women enrolled in a methadone maintenance program between 1994 and 2003. Groups were compared on demographic characteristics, psychiatric comorbidity, urinalysis results and retention rates. Groups were comparable in terms of most demographic characteristics and severity of addiction at intake. Groups did not differ significantly in terms of urinalysis results or retention rates. While most women reduced their drug use, a majority of both groups continued to use illicit drugs at least occasionally. Psychiatric comorbidity was significantly different with the non-pregnant group being more psychiatrically disordered. Clinical implications are discussed.