Objectives Risk judgements are key factors in adolescents’ decisions related to sexual health. We examine the associations between self-perceived risk and prevalent chlamydia infection, and sexual behaviours related to risk perception in a general adolescent population in Norway.

Methods Population-based cross-sectional study among 1028 sexually experienced girls and boys, age 15–20 years (85% participation), including web questionnaires and urine samples for Chlamydia trachomatis PCR testing. Participants rated self-perceived risk as: no/low/medium/high/very high. We used binary and ordinal logistic regressions to examine associations with chlamydia prevalence and self-perceived risk, respectively, adjusting for potentially confounding variables.

Results Chlamydia prevalence increased with increasing risk perception. Although girls had twice the chlamydia prevalence of boys (7.3% vs 3.9%), their risk distribution was similar and 65% of both genders rated their risk as no/low with half of infections detected in this group. In multivariable analyses, reporting multiple sexual risk behaviours, non-steady relationship, previous chlamydia testing and treatment, and urogenital symptoms increased self-perceived risk. More boys overestimated their personal risk whereas more girls underestimated it (52% vs 30%, respectively, and 15% vs 31%, p<0.001). The main reasons for perceiving no/low risk were: ‘I have a steady partner’ and ‘I trust my partner will tell me about an infection’.

Conclusions These sexually experienced adolescents acknowledged their chlamydia infection risk, but wrong beliefs were incorporated in their assessments, and more than half had incorrect risk perception. We suggest that sexually transmitted infection prevention programmes should be directed at closing the gap between perceived and actual risk and focus on how context may bias personal judgement.