The study tested whether children's prosocial behavior was negatively or positively related to children's and mothers' psychological problems. Participants were 149 London families when mothers were pregnant and followed up in infancy and at ages 4 and 11. Children's cooperation at 4 and general prosocial tendencies at 11 were negatively associated with externalizing problems but unrelated to internalizing problems. A subgroup of children who were more prosocial than average expressed clinically significant worries about family members. Maternal depression decreased prosocial behavior in the eyes of adults, but children of depressed mothers saw themselves to be prosocial. Early cooperation protected children against later risk for externalizing problems, even when their early behavioral problems were taken into account.