We studied the behaviour and lifestyle of adolescents, focused on the relationship between bullying and the underlying mental condition, using the SDS (self-rating depression scale), in 3,842 young people aged between the 5th and 9th grades of the Japanese education system. In both perpetrators and victims of bullying the scores tended to be high. Five point four percent of the group replied that they were bullied more than once a week and 3.2% admitted to bullying more than once a week. Forteen percent of the subjects stated that they had bullied others and had experienced being bullied. Bullying differed between males and females. The commonest form of bullying between males was physical whereas in females it was psychological. The following factors were noted in both bullies and bullied: teachers did not treat them fairly, friends were not helpful, the classmates did not accept them, and they often ended up alone. To evaluate mental health, fifteen SDS questions, from an original twenty, were selected for this study. The result displayed considerably high scores in both sexes and across all age groups. The most significant relationship of SDS scores to other factors was observed in bullying behaviour.