OBJECTIVE: To determine if a nurse-led or psychologist-led parent-training program was more successful than a minimal intervention in treating early childhood Oppositional Defiant Disorder (ODD) in pediatric primary care. METHODS: Twenty-four practices were randomized to conditions in which parents of 117, 3- to 6.11-year-olds with ODD received the 12-session Webster-Stratton Incredible Years program led by primary care nurses or clinical psychologists, or to a minimal intervention group in which parents received only the companion book to the treatment program. RESULTS: There was improvement across posttreatment and 12-month follow-up for all groups, but no overall treatment group effects. There was a dose effect, with a reliable, clinically significant gain after seven sessions on the Eyberg intensity scale, and nine sessions on the Child Behavior Checklist externalizing scale. CONCLUSIONS: There is little advantage to the therapist-led treatment over bibliotherapy unless parents attend a significant number of sessions.