Research has suggested that didactic and values discussions, life skills training, social skills training, and family intervention can reduce heavy drug use. Because subjects display variability in patterns of behavior change, increased understanding of these individual differences could be beneficial to interventions. A preventive intervention program was established for families concerned about their adolescents' alcohol or drug use. Adolescents provided data on daily drug, alcohol, and cigarette use for an 18-month period, along with school report cards and periodic self-report inventories. The intervention consisted of (1) identifying undesirable events; (2) assessing antecedents and consequences through weekly meetings; (3) focusing on the complaints; (4) teaching the family problem-solving techniques; and (5) encouraging consistent contingencies and modeling desired behaviors. The first two adolescents in the program showed widely differing patterns in almost every index except outcome; they both showed improvement by the end of follow-up in the two targeted behaviors (school performance and drug use) and showed no change in the two control, non-targeted behaviors (alcohol and cigarette use). (A detailed analysis of the behavior change patterns of these two subjects and the concurrent changes in self-efficacy expectancies, outcome expectancies, and values that might account for their differences is provided in the text and accompanying figures.) (NRB)