This paper applied a hierarchical linear modeling approach to explore the interaction effects of treatment program and client characteristics on client retention in treatment for drug users. Program characteristics included services provision, funding sources, and staff-client gender congruence, and client characteristics included gender, age at admission, and drug use level prior to admission. The same model was applied separately to three modalities: residential, methadone maintenance, and outpatient drug-free programs. Data were obtained from 59 treatment programs and 3,764 of their clients who had discharge records. The most noteworthy significant interaction effect detected was program's funding source and client's gender on treatment retention in the outpatient drug-free modality. For example, female clients remained less time in the programs that accepted only public funding than in the programs that accepted both public and private funding. Male clients remained in the treatment an average of 25.3 fewer days than female clients in drug-free programs that only accepted public fund, but stayed about the same time as females if the programs received mixed funding.