Substance abuse is a frequent and complicating feature of homelessness. Barriers to service access have prevented homeless individuals from receiving substance abuse care. Outreach to homeless persons is a mechanism for identifying homeless substance abusers and engaging them in treatment. This study describes a program of substance abuse treatment conducted by a medical care provider for homeless persons in Baltimore, Maryland and compares characteristics of outreach recipients to those of walk-in clients. The study also examines the differences in drug abuse pathology and selected treatment outcomes among homeless and non-homeless clients and the extent to which homeless persons are unidentified in the drug abuse treatment reporting system of the state. Homelessness is about 5.5 times more prevalent in the clientele of drug abuse treatment programs than is reflected in the statewide substance abuse management information system of Maryland. Composite scores on the Addiction Severity Index for homeless individuals are significantly higher (reflecting more serious problems) on every measure in the interview, compared to non-homeless individuals. Except for residential treatment settings, homeless persons demonstrate a shorter length-of-stay in substance abuse treatment than housed clients. Our outreach effort successfully located its targets. Outreach clients reported significantly higher levels of substance abuse than walk-in clients. In addition 42.3% of outreach contacts became service recipients. These findings indicate that outreach can be a successful method of targeting and engaging a segment of homeless substance abusers who are otherwise difficult to engage in treatment.