Previous studies of the characteristics of juvenile sex offending have been restricted to clinical samples (with convicted offenders undergoing treatment or assessment), which have several inherent limitations. To overcome these limitations victim allegations made to the London Metropolitan Police, UK, in 2001, of sexual assault by juvenile stranger perpetrators were sampled. The study aimed to determine more accurately the alleged nature of sex offending by this type of juvenile offender (aged less than 18 years) and to determine what factors were associated with the most serious cases of this type of aggressive behaviour. The most common offence characteristics were identified, and relationships between suspect and offence/victim characteristics were statistically tested. Offence seriousness, as indicated by physical violence and the occurrence of penetration, was significantly associated with an assault by a team of offenders, a young victim, and older juvenile offenders. The potential for predicting offender characteristics from offence characteristics was examined. Estimated offender age was significantly predicted by the absence/presence of penetration. An older juvenile offender was predicted by a penetrative attack. Aggr. Behav. 30:243–253, 2004. © 2004 Wiley-Liss, Inc.