To identify variables that discriminate needle-sharing among drug abusers, 224 male drug abusers were studied. They had been admitted to a 30-day inpatient drug treatment program over a 19-month period (September 1983 through March 1985). The variables examined were divided into three categories: demographic (age, race, education), personality (Minnesota Multiphasic Personality Inventory [MMPI] scores and MMPI deviant scores), and drug use patterns (drug of choice, use of single or multiple [mixed] drugs, severity of drug use, and place of use). Three variables were identified that discriminated needle-sharers from other drug abusers. Compared with other drug abusers, needle-sharers used more multiple drugs, were more likely to use a "shooting gallery," and had more problems related to drug use. No demographic or personality variables discriminated needle-sharers from nonsharers. The data suggested that needle-sharing is widespread in the drug culture. Needle-sharing was not confined to a particular racial group, educational level, or personality type. These findings can be used to structure education programs about acquired immunodeficiency syndrome (AIDS) for drug abusers. Drug treatment programs appear to provide an important opportunity to educate drug abusers about AIDS and related health issues associated with needle-sharing.