Ability-grouping has been studied extensively in relation to children's academic, but not emotional and behavioral outcomes. The sample comprised 7259 U.K. children (50% male) with data on between-class and within-class ability-grouping at age 7. Peer, emotional, hyperactivity, and conduct problems were measured at ages 7, 11, and 14 years. Children in low within-class ability groups showed more hyperactivity and emotional problems across the study period compared to non-grouped children, after adjustments for the different types of ability grouping and confounding. Additionally, children in the middle within-class ability groups showed more, and those in the top within-class groups less, hyperactivity compared to non-grouped children, after adjustment. Children in lower within-class groups should be monitored closely to ensure that their well-being is not compromised.