This report presents data on the empirical relationship between aggressiveness at the ages of 10-13 and criminal behavior in adolescence and early adulthood. This represents the first step in a process in which psychological, physiological, and environmental factors will be investigated as determinants of prosocial and antisocial behavior in a longitudinal perspective. The study is based on data for subjects born in 1955, who were all male school children in a town of about 10,000 inhabitants and in the 3rd grade. The specific investigation reported here was performed on those boys for whom data on aggressiveness were obtained both at the age of 10 and at the age of 13 (n=412). Ratings on aggressive behavior were obtained from teachers, and information on registered law-breaking was supplied by the National Police Board. One out of five of the boys had at some time during adolescence committed one or more offenses recorded by the police. The study found that the higher the aggressiveness score, the higher the probability that a boy will be found in the police register. Boys who are highly aggressive in early grades commit more grave offenses. Factors to be considered in future research are discussed, and tabular data and 47 references are provided.