The data covered 9,945 boys born in 1945 and 13,160 born in 1958. Information came from school records, official records on delinquency, and interviews with an adult sample. Results revealed that 33 percent of the 1957 birth cohort and 34.9 percent of the 1945 cohort had at least one police contact before their 18th birthday. However, the 1958 cohort committed proportionally more crimes and much more serious crimes than the earlier cohort. In addition, a small fraction of the youths committed a majority of the delinquent acts. More than 60 percent of the first offenses were nonindex crimes. Delinquency was more prevalent among nonwhites and youths of lower socioeconomic status. Delinquency was also associated with residential instability, poor school achievement, and failure to graduate from high school. Results suggested the need for close probation supervision for second-time index offenders and incapacitation in a secure facility after the third index offense. In addition, improved handling of offenders within the juvenile justice system is preferable to the increasing tendency toward juvenile court waiver. Tables, index, and 106 references