The objective of the study was to examine the relationship between the perception of selected peer behaviours, social relationships and the neighbourhood social capital among 15-year-old adolescents in Poland. Research was carried out in 2010 as part of a successive series of the HBSC international studies on health behaviours in school-aged children (Health Behaviour in School-aged Children: AWHO Collaborative Cross-national Study). 1551 students in the third grade of junior high schools were surveyed (762 boys and 789 girls). The standard, international, anonymous HBSC questionnaire was used in the surveys. In the analyses the scales for perception of male and female risky peer behaviours, the scales for communication with peers as well as the scales of confidence in friends, social competences and the social capital were used. It was found that the perception among respondents of their male and female friends as individuals engaging in risky behaviours (tobacco smoking, alcohol consumption, getting drunk) was widespread. Almost 40% of respondents claimed that all or most of their friends smoked cigarettes; one in two respondents was of the opinion that they drank alcohol; and one in ten that they get drunk. In the analyses comprising linear regression it was demonstrated that as the intensity of risky peer behaviours increases, selected social relationships deteriorate; this particularly applies to the general assessment of social capital. With regard to preventive measures aimed at counteracting risky behaviours among adolescents more attention should be paid to compliance with the provisions of law on tobacco smoking and alcohol consumption by minors among private individuals as well as those professionally dealing with law enforcement. An important task is also to raise the awareness of adolescents and adults of the relationship between young people engaging in risky behaviours and declining confidence in individuals and formal and informal institutions.