A comprehensive self-report questionnaire that covered many aspects of adolescents' lives was administered to 1270 students in Years 7, 10 and 11 who attended seven State high schools in Sydney. The prevalences of unhealthy and acting-out behaviours increased significantly between the ages of 12 and 15 years (Years 7 and 10) and then levelled out. By the age of 15 years, 15% of adolescents were drinking alcohol at least three times a week; 30% of adolescents smoked cigarettes; 15% of adolescents had used marijuana in the past week; and 18% of adolescents had had previous sexual experience. Nine per cent of all adolescents often felt "so down or low that life has lost its meaning". The quality of the family relationship was found to be associated closely with the presence or absence of adolescent depression and unhealthy or acting-out behaviours. Where the family relationship was not close or loving, the prevalence of depression was three times as high and the prevalences of alcohol, cigarette and marijuana use and early sexual activity were about twice as high as among adolescents who perceived that their families were close and loving. The majority of adolescents with poor family relationships felt that their parents could do more to help them. Help in the form of more understanding, support, trust, time, interest and freedom was wanted much more frequently than was help in the form of material items such as pocket-money. Parental divorce or separation and having a mother who was working full-time were also associated with a perceived lack of family closeness and love. The implications of these findings for the prevention of the establishment of health-threatening behavioural patterns and suicide among adolescents are discussed.