The aim was to examine the relationship between prosocial behavior and conduct problems, especially aggression, in early childhood. In Phase 1 of the study, teachers reported on 93 3-5-year-old children's prosocial behavior and psychological problems, using a screening instrument, the Strengths and Difficulties Questionnaire (SDQ). In Phase 2, 65 parents completed the SDQ and were interviewed in depth, using a clinical interview, the Preschool Age Psychiatric Assessment (PAPA). Children with clinically significant symptoms of conduct disorder and ADHD were significantly less prosocial than other children. However, aggressive conduct symptoms were unrelated to prosocial behavior, once symptoms of ADHD were taken into account. The findings suggest that problems in regulating activity and attention interfere with prosocial development. (Contains 5 tables.)