This study set out to assess the relationship between pubertal timing and emotional and behavioural problems in middle adolescence. The study involved a school based survey of health, health behaviour and behaviour in school as well as questions about emotional and behavioural problems (the School Health Promotion Study). Secondary schools in four regions and 13 towns in Finland participated in the study in 1998. The respondents were 36,549 adolescents aged 14-16. The study included questions on depression, bulimia nervosa, psychosomatic symptoms, anxiety, drinking, substance use, smoking, bullying and truancy. Among girls, both internalising and externalising symptoms were more common the earlier puberty occurred. Among boys, externalising symptoms only were associated with early puberty. It is concluded that early pubertal timing is associated with increased mental health problems. Professionals working with adolescents should consider the mental health needs of early maturing adolescents.