Applying the Theory of Planned Behavior (TPB) to the process of engagement in preventive parenting groups, we tested the ability of family and child measures to predict intent to enroll, enrollment, attendance, and quality of participation in PACE (Parenting Our Children to Excellence). PACE is a prevention trial testing the efficacy of a structured program to promote effective parenting and reduce risk of adverse child outcomes. Mothers of preschoolers (N = 451) from diverse ethnic and socioeconomic backgrounds participated at two sites. Results showed that mothers who stated their intent to enroll reported relatively few time constraints but high levels of stress, as did mothers who enrolled. The latter also experienced elevated levels of oppositional defiant child behaviors, indicating that the program reached families who stood to benefit from it. Attendance, which was also best predicted by few time constraints, was high (with 49% of mothers who enrolled attending 5 or more of 8 sessions). In turn, attendance predicted quality of participation (at 1 site only), with mothers attending more sessions participating more actively and enthusiastically. Ethnicity and, with a few exceptions, socioeconomic circumstances and site, were not significant predictors of intent, enrollment, or attendance. Results provide qualified support for the TPB and illustrate its relevance to preventive research and interventions.