A survey of 481 homeless persons in Cook County, Illinois, was conducted to assess the prevalence of alcohol and illicit drug use in this population, and potential treatment needs. Respondents were sampled at random in emergency and transitional shelters, soup kitchens, drop-in centers, and single room occupancy (SRO) hotels. Approximately 30% were characterized as having alcohol-related treatment needs. A slightly smaller proportion (26.7%) were estimated to have treatment needs associated with drug use behavior, and 13.5% were found to have both alcohol and drug-related, or dual, treatment needs. Those with alcohol user treatment needs were mostly male, age 35 and older, White, and living in SRO hotels. In contrast, persons with drug user treatment needs found in shelter settings. Implications for the treatment of homeless persons with substance misuse problems are discussed.