A significant percentage of children with disruptive behavior disorders do not receive mental health assistance. Utilization is lowest among groups whose children are at greatest risk. To increase the availability, accessibility, and cost efficacy of parent training programs, this prospective randomized trial compared a large group community-based parent training program to a clinic-based individual parent training (PT) programs. All families of junior kindergartners in the Hamilton public and separate school boards were sent a checklist regarding problems at home. Those returning questionnaires above the 90th percentile were block randomly assigned to: (1) a 12-week clinic-based individual parent training (Clinic/Individual), (2) a 12-week community-based large group parent training (Community/Group), or (3) a waiting list control condition. Immigrant families, those using English as a second language, and parents of children with severe behaviour problems were significantly more likely to enroll in Community/Groups than Clinic/Individual PT. Parents in Community/Groups reported greater improvements in behaviour problems at home and better maintenance of these gains at 6-month follow-up. A cost analysis showed that, with groups of 18 families, Community/Groups are more than six times as cost effective as Clinic/Individual programs.