Aim. An assessment of the health status in adolescence includes, among other variables, risk behaviours that may involve either direct or potential mental health risk. In the study two categories were introduced in the mental health assessment, defined as externalising (problem behaviour) and internalising (emotional disturbances) indicators. The first aim of the study was to estimate problem behaviour prevalence among students beginning secondary school, while the second objective was to analyse the relationships between internalisation and externalisation indicators. Material and method. The participants of the study were first grade students (N = 1123) of secondary schools in the City of Warsaw area. They responded to a Polish adaptation of a self-report Canadian questionnaire monitoring the adolescents' mental health. The following externalising indicators of risk behaviours were used: getting drunk, problems due to alcohol drinking, drug use, problems caused by drug use, violence, law-breaking. The following internalising indicators were analysed: depressive symptoms (as measured by the CES-D scale), psychological distress (the GHQ-12 questionnaire by Goldberg), selfrated poor mental health, suicidal thoughts. Results. The presence of at least one of the risk behaviours was reported by a half of the sample (52%), more often by boys (59.9%). A high percentage of those manifesting problem behaviour were characterised by a higher intensity of experienced psychological stress, more severe depressive symptoms and worse self-rated psychological functioning. Those who reported symptoms of poor mental health, together with two or more problem behaviours constituted 14.9% of the sample. Conclusion. The group at risk for mental health constituted about a third of the sample studied, irrespective of gender.