Combining local arrest data and drug use rates among arrestees in Los Angeles County, a methodology was developed to provide estimates of the number of drug users in several categories among CJS-involved populations. In 1989, Los Angeles County had an estimated 145,395 arrestees who used cocaine, 39,633 who used heroin, 193,883 who used some illicit drug, and 67,839 who used drugs by injection. Estimates of drug use in the overall estimated criminal population indicate 371,730 cocaine users, 109,172 heroin users, 504,738 users of some illicit drug, and 190,505 users of drugs by injection. About 60 percent of a sample of arrestees who showed recent use of at least one illicit drug reported the need for drug treatment. In contrast, only 14,044 injection drug users and 5,356 cocaine users were admitted to treatment in Los Angeles in 1989. Relative to their counterparts, arrestees who were white, female, and older reported a higher need for treatment. Suggested strategies for meeting offender treatment needs are categorized in terms of preincarceration, inmate programs, and community aftercare. 53 references and 1 table (Author abstract modified)