Index offenses and offense histories and seriousness were studied. An examination of the relation among such background variables as race, socioeconomic status (SES), types of schools attended, residential and school moves, highest grade completed, and IQ achievement level revealed that the variables of race and SES were most strongly related to the offender-nonoffender classification. Whites (71 percent of the cohort) were involved in 44 percent of the 10,214 delinquent acts, while nonwhites (29 percent of the cohort) committed 56 percent of these offenses. The nonwhites' offense rate was 3.1 times as high as the white rate. Among Index offenses, the nonwhite crude rate is 2.4 times as great, but the seriousness weighted rate is 4.6 times as great. However, knowledge of the number and type of previous offenses does not aid in predicting the type of offenses to be committed next; it appears that offenders 'start over' each time they commit an offense. There also appears to be no specialization in offense types. This knowledge of chronic offenders combined with an understanding of desistance rates (i.e., the number of delinquent acts falls after the third offense) illustrates the need for intervention programs after commission of the third offense. The interview followup study of a systematic 10 percent random sample (974 subjects) points to a 43 percent probability of having an official arrest record before reaching age 27. Moreover, 8 percent of the cohort who had no previous juvenile arrest record became adult offenders. However, the chance of becoming an adult offender is 3.5 times higher for persons who had at least one arrest under age 18. The peak probability of having a first arrest occurs at age 17, while the probability of ever being arrested, as well as of committing offenses generally declines with age, beginning with age 18. Further analyses will address (1) the desistance rate after one, two, and three offenses; (2) any continuation of the Markovian chain of offense specialization for adult offenders; and (3) the optimal point in adult offenders' lives when strong intervention should be undertaken to reduce further criminality and to better protect society. Tabular data and 11 notes are appended.