The study sample consisted of approximately 27,160 males and females born in 1958 who resided in Philadelphia from age 10 to age 18. By using information up to age 26 for the subjects, the study provided longitudinal observation of subjects' encounters with police and thereby identified the youths most likely to commit crimes as adults. Both the Sellin-Wolfgang severity scale and the Uniform Crime Index for each member of the birth cohort measured the overall severity of the delinquency career. The proportion of adult offenders whose delinquency careers fell within each seriousness level was compared to that for nonoffenders with similar juvenile experiences. Logistic regression determined the effect of serious delinquency on adult offending, with simultaneous consideration of race, gender, socioeconomic status, and age. Tabular analyses revealed that delinquents who became adult offenders by the age of 26 were somewhat more likely than other delinquents to have had more seriously offensive adolescent careers. This relationship remained consistent when race and gender subgroups were examined. 4 tables, 49 footnotes.