To determine the prevalence of substance use in adolescents with eating disorders, compare the results with a data set of Ontario high school students, and explore why adolescents with eating disorders do, or do not, use various substances. From January 1999 to March 2000, 101 female adolescents who met the DSM-IV criteria for an eating disorder were followed up in a tertiary care pediatric treatment center. They were asked to participate in a cross-sectional study using a self-administered questionnaire assessing substance use and investigating reasons for use and nonuse; 95 agreed to participate and 77 completed the questionnaire (mean age, 15.2 years). The patients were divided into two groups: 63 with restrictive symptoms only, 17 with purging symptoms. The rates of drug use between subjects and their comparison groups were compared by z-scores, with the level of significance set at.05. During the preceding year, restrictors used significantly less tobacco, alcohol, and cannabis than grade- and sex-matched comparison populations, and purgers used these substances at rates similar to those of comparison subjects. Other drugs seen frequently in the purgers included hallucinogens, tranquilizers, stimulants, LSD, PCP, cocaine, and "ecstasy." Both groups used caffeine and laxatives, but few used diet pills. Restrictors said they did not use substances because they were bad for their health, tasted unpleasant, were contrary to their beliefs, and were too expensive. Purgers generally used substances to relax, relieve anger, avoid eating, and "get away" from problems. Female adolescents with eating disorders who have restrictive symptoms use substances less frequently than the general adolescent population but do not abstain from their use. Those with purging symptoms use substances with a similar frequency to that found in the general adolescent population. Because the sample size for the purging group was small, firm conclusions cannot be drawn from our analysis. Health care providers who treat adolescents with eating disorders are in a good position to identify those who use substances and may be at risk for substance abuse.