The first study explored the delinquency careers of all boys born in 1945 who lived in Philadelphia between the ages of 10 and 18. Data were obtained on first offense probabilities, recidivism (especially chronic repeaters), offense switching rates, offense severity escalation, age at onset and offense accumulation, disposition probabilities, and subsequent offense behavior. A 1958 cohort study replicated the first study, except that it included females. In the first study, there was a 35 percent probability that the 10,000 boys had at least one police arrest for offenses other than traffic violations. Only 6 percent of the cohort were chronic offenders (had at least five official arrests prior to age 18), and they were responsible for well over 50 percent of all offenses. In the second study, the percentage of chronic offenders was 7.5, and they committed 69 percent of the index offenses. About 9 percent of the 13,800 boys in the second study and about 2 percent of the 14,500 girls committed a violent offense that resulted in injury to a person. The probability of committing a second injury offense is 18 percent for a white male, 38 percent for a nonwhite male, 5 percent for a white female, and 11 percent for a nonwhite female. The second cohort group was more delinquent in general than the first group and engaged in more injurious behaviors. They are more violently recidivistic. Tabular data are provided.