Objective: To study associations between emotional and behavioural symptoms and later engagement in sexual behaviours in middle adolescence. Materials and methods: All ninth graders in two Finnish cities were recruited to respond to a questionnaire focusing on mental health and disorders, health behaviours and risk and protective factors (T1), and a follow-up survey two years later (T2). The baseline sample (94.4% of all eligible) comprised 1609 girls and 1669 boys, with mean age of 15.5 years (SD 0.39). A total of 2070 adolescents completed the survey at both T1 and T2. The response rate of the final sample was 63.1% (2070/3278). Of the respondents, 56.6% were girls. Experience of intercourse and number of partners for intercourse were elicited at age 17. Depression was measured with the 13-item Beck Depression Inventory, conduct disorder with the Youth Self-Report, and smoking and alcohol consumption with questions widely used in Finnish adolescent health surveys. The data were analysed using cross-tabulation with chi square statistics for classified variables and t-test for continuous variables. Logistic regression was used to study multivariate associations. Results: Depression, conduct disorder, smoking and drinking at age 15 were associated with having experienced intercourse by age 17 (odds ratios (ORs) 1.8–10.3) and with multiple partners for intercourse by age 17 (ORs: 2.4–4.7) among girls. In boys, frequent alcohol use and smoking at age 15 (ORs: 2.2 and 4.6) were associated with experience of intercourse by age 17, and these and conduct disorders at age 15 with multiple partners for intercourse by age 17 (ORs: 2.8–3.2). Conclusion: Emotional and behavioural disorders in middle adolescence are associated with sexual activity and risk-taking sexual behaviours later in the developmental phase.