Sociometrics and a revised Pupil Evaluation Inventory (PEI) were administered to 362 boys in Grades 3-6 to define peer-rated narrow-band dimensions of undercontrolled behavior associated with peer rejection. Three dimensions resulted from factor analysis: aggression, hyperactivity, and inattention-immaturity. All 3 dimensions were associated with peer rejection; but only inattention-immaturity was negatively associated with positive sociometric status. Two distinct clusters of boys with undercontrolled behavior problems were found: a hyperactive and inattentive-immature group and a hyperactive and aggressive group. Results indicate that a more precise and powerful model of peer rejection in boys is obtained through the use of narrow-band dimensions of under-controlled behavior than through broad-band aggression-disruption scales from existing peer-rating measures.