The literature indicates that peer influence is important in understanding adolescent drug use. The nature of peer interactions predicts a high degree of similarity in drug use among friends. To test that hypothesis, a detailed analysis of this correspondence was conducted on a large sample of junior and senior high school students. Findings indicated that a youth who used specific drugs in the last thirty days almost invariably has friends who also use those same drugs, but is considerably less likely to have friends who use other drugs or no drugs. In addition, friends' use is strongly associated with the adolescent's use of marijuana, uppers, cocaine, downers, and PCP across five distinct drug use styles.