We conducted a cross-sectional study of the risk behaviours inherent in tobacco smoking, alcohol consumption and premarital sex, among 400 medical students (186 males) from a medical university, Yangon, and 410 community youths (244 males) 15 to 24 years of age from selected townships in Myanmar. As a result, we found that 12.8% smoked, 34.5% consumed alcohol and 10.1% engaged in premarital sex, among medical students, whereas among community youths, the corresponding rates were 28.8%, 32.1% and 11.9%. There was a significant difference in the prevalence of all risk behaviours between male and female respondents. Such risk behaviours were more dominant among males, while being very low among females. Among male respondents, the smoking rate was significantly higher among community youths (46.7%) than among medical students (26.9%); however, student alcohol consumption (58.5%) was greater than that of community youths (47.1%). Premarital sexual experience did not differ significantly between the two groups. These risk behaviours were correlated with one another. Having close friends who engaged in similar behaviours was found to be the major contributing factor for those kinds of risk among both groups. Our results highlighted the fact that, despite their relatively sophisticated knowledge of risks, the prevalence of risky behaviour among the medical students was no less frequent than among community youths. To diminish those risks, evaluations of actual conditions, behaviour modifications and specific preventive measures compatible with existing culture and changing lifestyles should be undertaken. Effective adolescent health programs at schools, colleges and universities should be revised and emphasized.