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Substance abusers, especially cocaine abusers, may underreport their substance use in outcome interviews. Follow-up interviews were conducted and urine specimens were obtained on 633 persons 9 months after admission to a 3-month cocaine treatment program. Although 422 (67%) reported no use of cocaine in the past 30 days, 134 of these (32%) had cocaine-positive urines. This group did not differ on most characteristics at intake or follow-up from the 288 with cocaine-negative urines. The amount of treatment received did affect willingness to admit drug use. Of 132 treatment completers who reported no cocaine use at follow-up, 21 (16%) had positive urines. Of 91 early dropouts who also reported no cocaine use, 36 (40%) had positive urines. This differential rate of underreporting had the effect of seriously underrepresenting the effectiveness of treatment completion as compared with little or no treatment.