Utilizing data collected in the Cambridge Study in Delinquent Development which is a prospective longitudinal survey of London boys from aged 8 to 48, this study investigated whether risk factors for offending were similar or different for males and females. The main comparison was between the brothers and sisters of the boys in the Cambridge Study. The objectives of the research were to investigate similarities and differences in childhood risk factors for offending boys and girls, compare criminal careers of males and females, and to draw out implications for future research, policy, and practice and especially for risk-focused prevention. Results indicate that the prevalence of convictions was greater for brothers than for sisters and that brothers committed offenses more frequently. Brothers and sisters were more similar in ages of offending. Their average age of offending were almost identical at 21 years of age. In general, the most important risk factors for offending, for early onset (before age 17), and for frequent offending were similar for brothers and sisters and included: low family income, large family size, attending a high-delinquency rate school, a convicted father, a convicted mother, a delinquent sibling, parental conflict, separation from a parent, harsh or erratic parental discipline, and poor parental supervision. However, some important differences were socio-economic risk factors, such as low social class, low family income, poor housing, and large family size which predicted offending more strongly for sisters than for brothers. Future needed research and policy implications for risk-focused prevention are briefly discussed. Tables and references