168 male and 142 female 18-27 mo old children were observed and rated on problem behavior as they entered peer play groups consisting of 12-15 Ss and 2-3 caregivers. A schedule was used that allowed the observer to code the S’s behavior and the reaction of peers and caregivers to that behavior. Four categories of child problems were selected for study: conduct problems with aggression, withdrawn undersocialized behaviors, separation anxiety, and dependent personality. Three questions were asked concerning the persistence and consequences of problem behaviors in Ss: Do the behaviors persist over a period of 1-2 yrs? What kinds of reactions does an S who is engaged in these behaviors receive from peers and teachers? How do the reactions received from peers and teachers of problem Ss compare to those of the average of their own class? Results show that the stability of problem behaviors and reactions received by Ss were related to S’s gender and to the sex appropriateness of the behavior. The chance that behavior problems would continue was found to be greater for males who produced a high number of conduct behavior problems. Teachers responded to reinforced males’ aggression more than that of females. Teachers responded to dependent males more negatively, while they responded to dependent females more positively.