This longitudinal study assessed the role of afterschool program (ASP) participation in the development of child obesity and peer acceptance in a sample of 439 children. Most participants lived in poverty and were Hispanic or African American. Measurements of height and weight determined obesity status and peer acceptance was assessed through teacher and peer responses to written surveys. Rates of obesity were 22% at baseline (M age = 4.9) and 29% at follow-up (M age = 8.1). Peer acceptance was significantly lower for obese children than nonobese children. Children who became involved in ASPs were significantly less likely to be obese at follow-up than nonparticipants. Both obese and nonobese ASP participants showed significant increases in peer acceptance over time. (Contains 6 tables, 1 footnote, and 3 figures.)