Adolescents' health-endangering behaviours receive attention because they are presumed to threaten the health of individuals in either the short or long term. The present study examined the role of psychosocial determinants on adolescents' health-endangering behaviours using elements of a biopsychosocial model proposed by Irwin and Millstein (1986). It was hypothesized that egocentrism, self-esteem, and perceived social environment affect the onset of risk-taking behaviours, mediating risk perception. Eight hundred and eight Japanese college students completed questionnaires. Results from a structural equation analysis partly supported the hypothesized model. Egocentrism contributes directly to health-endangering behaviours while influences of self-esteem and perceived social norms are mediated by risk perception.