Background: Adolescent behaviour is strongly linked to emotions. The aim of this study is 1) analyse the differences between young offenders and non-offenders in emotional instability, anger, aggressive behaviour, anxiety and depression, and also the differences according to sex; and 2) compare the relation between emotional instability and anxiety, depression and aggressive behaviour mediated or modulated by anger in both groups. Method: participants are 440 adolescents, both male and female (15-18 years old). 220 were young offenders from four different correctional centres of the Valencia Region. The other 220 participants were randomly chosen from ten public and private schools in the Valencia metropolitan area. In the Schools the instruments were applied collectively in the classroom, with a 50 minutes maximum duration. In the Youth Detention Centre the application was carried out in small groups. Results: The structural equation model (SEM) carried out on each group, young offenders and non-offenders show a relation between the assessed variables. Emotional instability appears strongly related with anger in both samples, but anger just predicts depression and aggressive behaviour in the offender population. Conclusions: The results give relevant information for treatment and prevention of aggressive behaviour and delinquency in teenagers through emotional regulation.