Data on aggressive contacts were collected from a sample of 62 delinquent boys and a control group of 58 lower class and 45 upper middle class nondelinquent high school students. Additional data on injuries received, inflicted, and both; of weapons used; seriousness of injury; and other variables were collected. Statistically significant differences were found between delinquent and nondelinquent boys in number of fights and number of injuries, and also between lower and middle class boys. Nearly all boys reported fighting opponents; the more opponents, the more involvement of weapons, multiple injuries, and charged offenses. Similar associations were drawn in frequency of fighting in the home and in late age of maximum fighting. Delinquent boys found most opponents in the street, compared to nondelinquents who fought among school peer groups. The five delinquent boys with any sign of mental disturbance showed a lower frequency of fighting than the rest of the sample. However, there were some in the total sample which could be used to identify delinquency and which were not found among the nondelinquent group. These predictors are the reporting of more than 65 fighting opponents, the receipt or infliction of more than one serious injury, and the rise of weapons. The article proposes a relationship between aggressive childhood behavior in the home and later violence, and also sugests that adolescent violence in the home is likely to be viewed as a sign of mental illness, whereas violence against strangers is perceived as criminal behavior.