Pregnant substance users can benefit significantly from substance abuse treatment, but treatment retention can be challenging. Two hundred pregnant substance users entering outpatient substance abuse treatment at one of four treatment programs were randomized to receive either three individual sessions of Motivational Enhancement Therapy for pregnant substance users (MET-PS) or the first three individual sessions normally provided by the program. All participants were encouraged to participate in all other treatment offered by the program. Outcome measures included treatment utilization according to clinic records, qualitative urine toxicology measures, and self-report of substance use. One hundred sixty-two (81%) participants completed the 1-month active phase. Participants attended 62% of scheduled treatment on average and reported decreased substance use during the first month of treatment, with no differences between MET-PS and treatment-as-usual (TAU) participants. There was some evidence that the efficacy of MET-PS varied between sites and that MET-PS might be more beneficial than TAU in decreasing substance use in minority participants. These results suggest that MET-PS is not more effective than TAU for pregnant substance users in general but that there might be particular subgroups or treatment programs for which MET-PS might be more or less effective than TAU.