A structured interview was developed to assess drug use behavior, criminal behavior for a range of offenses, physical and psychological health, social situation, and treatment history. Data were collected on self-reported involvement in and frequency of drug selling activities and other crimes during the 90-day periods prior to each interview. Most drug misusers were men (73 percent) and white (92 percent), and the average age was 29 years. A huge number of drug selling offenses (39,153) was reported prior to intake. Fewer than one-third of respondents reported having sold drugs during the 90 days prior to intake; among those who sold drugs, drug selling was an infrequent and occasional activity. Most respondents reported not selling drugs. A small minority of respondents (7 percent) committed the majority of drug selling offenses (89 percent). These high-rate drug sellers reported different patterns of drug misuse to other drug sellers, including more frequent use of heroin but less severe dependence on heroin and less frequent drinking. The involvement of high-risk drug sellers may have reflected a more "professional" approach to dealing. Reductions in dealing were found for clients from both residential and methadone drug treatment programs. Overall, the number of dealing offenses at 1 year was reduced to less than one-fifth of intake levels, and the rate of involvement in crime was also reduced to less than two-thirds of intake levels. Reductions in drug selling were associated with reductions in regular heroin use. 37 references, 2 tables, and 1 figure