The retention of patients (n = 399) enrolled in mobile health services (MHS), a Baltimore outpatient mobile methadone treatment program, was compared to patient retention (n = 1588) in six Baltimore fixed-site programs. Mobile program patients were retained for a median of 15.53 months in treatment in comparison to 3.90 for fixed-site patients (n = 664) from the MHS served zip codes (MHSZIPS) and 6.27 for fixed-site patients (n = 924) from zip codes other than those served by MHS (OTHERZIPS), (P < 0.001). Using Cox regression, the characteristics of patients associated with earlier discharge were (1) higher number of arrests, (2) more frequent cocaine use and (3) lower family income. These predictors of shorter retention were generally more prevalent among patients from the MHS served zip codes. Therefore, the longer retention in treatment of MHS patients as compared to OTHERZIPS fixed-site program patients is even more striking. Consistent with these differences in retention, were finding in a prior study suggesting that the mobile program provided greater access to services in reducing patient transportation cost and travel time. Thus, mobile methadone maintenance treatment appears to be a useful means of providing services.