In the Cambridge Study in Delinquent Development, over 400 London males have been followed up from age 8 to age 48 in face-to-face interviews and up to age 56 in criminal records. About 42 % of the males were convicted up to age 56. During five age ranges up to age 47, 94 % of the males admitted at least one of eight offenses, in comparison with 31 % who were convicted for at least one of these offenses in these age ranges. The prevalence of offending, and the number of offenses committed, decreased steadily after age 18 according to both convictions and self-reports. On average, there were 38 self-reported offenses per conviction, and this ratio also decreased with age. Convicted males self-reported 25 offenses per conviction on average. It is concluded that the “scaling-up factor” from convictions to self-reported offenses is very important, especially in evaluating the effectiveness of intervention programs.