The authors review the literature on the high prevalence but underrecognition of substance abuse among hospitalized patients and the general unavailability of hospital-based substance abuse consultation services. They describe the development, clinical operations, staffing, and teaching activities of a large substance abuse consultation service in one urban academic medical center and detail the service's growth and changing utilization patterns over an 8-year period, reporting the clinical characteristics of 1,819 patients seen over a 1-year period. Differences in patient demographics, patterns of substance use, and diagnoses were highly significant from one referring service to another. The authors discuss the implications of such a service for patient care and teaching.