OBJECTIVE: To examine the extent to which linkage mechanisms (on-site delivery, external arrangements, case management, and transportation assistance) are associated with increased utilization of medical and psychosocial services in outpatient drug abuse treatment units. DATA SOURCES: Survey of administrative directors and clinical supervisors from a nationally representative sample of 597 outpatient drug abuse treatment units in 1995. STUDY DESIGN: We generated separate two-stage multivariate generalized linear models to evaluate the correlation of on-site service delivery, formal external arrangements (joint program/venture or contract), referral agreements, case management, and transportation with the percentage of clients reported to have utilized eight services: physical examinations, routine medical care, tuberculosis screening, HIV treatment, mental health care, employment counseling, housing assistance, and financial counseling services. PRINCIPAL FINDINGS: On-site service delivery and transportation assistance were significantly associated with higher levels of client utilization of ancillary services. Referral agreements and formal external arrangements had no detectable relationship to most service utilization. On-site case management was related to increased clients' use of routine medical care, financial counseling, and housing assistance, but off-site case management was not correlated with utilization of most services. CONCLUSIONS: On-site service delivery appears to be the most reliable mechanism to link drug abuse treatment clients to ancillary services, while referral agreements and formal external mechanisms offer little detectable advantage over ad hoc referral. On-site case management might facilitate utilization of some services, but transportation seems a more important linkage mechanism overall. These findings imply that initiatives and policies to promote linkage of such clients to medical and psychosocial services should emphasize on-site service delivery, transportation and, for some services, on-site case management.