The juveniles (575 in all) were followed up during a total of 10 years. A sociometric method based on police data was developed for studying co-offending patterns. The method was validated through a smaller number of interviews with the juveniles. This study shows that a large majority of the most delinquently active juveniles was associated with a large network of delinquent relations. The stronger the connection to this network, the more substantial, serious, and long-lasting were the delinquency and asociality. The network also seemed to be very important with regard to the reproduction of juvenile delinquency in the municipality. At the end of the follow-up period juvenile delinquency in Borlange had basically the same character as at the beginning, even though the juveniles in the network were, to a large extent, new. The young offenders who were central figures in the new network had successively learned their roles from the network's old members. 4 figures, 2 tables, and 36 references. See NCJ 124605-124612 for individual papers. (Author abstract)