Low rates of participation constitute a serious problem for family-based drug-use prevention programs. This study analyzes the characteristics of calls for parents' participation in such programs through manipulation of the variables indicated by the Spoth and Redmond (1995) model as involved in the process (severity of the drug problem, susceptibility of the family's own children and existence of barriers to participation). Participants were 485 school pupils ages 12 to 14 years and their respective parents. Results show that the attempt to manipulate such variables did not have behavioral consequences in relation to participation in the family-based prevention program. There is a need for strategies that increase attendance of high-risk families in this type of intervention. The study's limitations are noted.