Objective: To examine cross-sectional and longitudinal associations between behavioral problems and weight status, considering body mass index (BMI) z-scores and overweight status, in a community sample of preschoolers. Methods: The Strengths and Difficulties Questionnaire and the Diagnostic Interview for Children and Adolescents were administered to 611 parents. Adjusted general linear models and binary logistic regressions were used. Results: Children who were overweight and had a higher BMI were at increased risk of peer problems and attention-deficit/hyperactivity disorder (ADHD) symptoms. Prospective analyses showed that a higher BMI at the age of 3 years was predictive of peer problems at ages 4 and 5 years and hyperactivity and ADHD symptoms at the age of 4 years. Conclusion: This is the first study using a diagnostic- based instrument that shows a relationship between weight status and ADHD symptoms in preschoolers. Overweight children might benefit from screening for behavioral disorders and peer relationship problems.