The study examined the patterns of opiate use of clients enrolled in a Denver outpatient clinic by means of the presence of opiate metabolites in their urine samples collected before and after they were arrested for possession. The clinical course of a sample of clients who abused opiates before but not after the arrest was also analyzed. In addition, urine data from the other two methadone programs in the city were examined. The impact of the police action against drug dealers was clearly reflected in the decrease in opiate urine samples collected at the clinic. Urine data from both of the other clinics also surprisingly showed the same steep decrease in detected opiates following the police action. A striking aspect of the clinical information is that clinic personnel were unaware of the radical change in client opiate use before the study data were compiled. This may have been due to the fact that the urinalysis data come to the counselors in batches several times each week and are difficult to analyze in this fragmented form. Also, the clinic focuses on resolving emotional, interpersonal, and employment problems of clients, which may detract from a systematic analysis of clients' drug use. A greater emphasis on reducing drug use while dealing with other client problems might be a more effective approach. Clearly, the police action did reduce the quantity of drugs available to abusers, thus aiding treatment efforts. This result is particularly evident when an area's drug network is isolated, as is the case with Denver. Graphic data and one reference are provided.