Act 165 classifies "dangerous juvenile offenders" as youth 15 years old or older who have committed various specified serious offenses. A total of 592 juveniles arrested in Philadelphia in 1975 provided a population of juveniles divided into three distinct groups. The groups were juveniles with at least two delinquency adjudications for an Act 165 offense, juveniles with only one adjudication for an Act 165 offense, and juveniles with at least one adjudication for burglary with no history of arrest as a juvenile for any Act 165 offense. The average age at onset of juvenile criminal activity was 13 years old, and the study population accounted for over 3,600 juvenile arrests and over 2,600 adult arrests during the study period. Eighty-eight percent of the juveniles with at least 2 adjudications had at least six juvenile arrests, and 53 percent of the remaining juveniles had at least six juvenile arrests. A small percentage of juveniles committed only violent crimes. 7 tables, 5 figures.