||OBJECTIVES: Examining the associations between health and lifestyle factors recorded in the participants' early teens and development of suicidal thoughts recorded 4 years later. DESIGN: Population-based prospective cohort study. SETTINGS: All students in the two relevant year classes in Nord-Trondelag County were invited, 80% attended both waves of data collection. PARTICIPANTS: 2399 secondary school students who participated in the Young-HUNT1 study in 1995-1997 (13-15 years old) were included in a follow-up study 4 years later (17-19 years old). PRIMARY OUTCOME MEASURE: Suicidal thoughts reported at age 17-19 years. RESULTS: 408 (17%, 95% CI 15.5% to 18.5%) of the adolescents reported suicidal thoughts at follow-up, 158 (14.2%, CI 13.6% to 16.4%) boys and 250 (19.5%, CI 18.8% to 22.0%) girls. Baseline anxiety and depressive symptoms (adjusted OR (aOR) 1.9, CI 1.4 to 2.6), conduct problems (aOR 1.8, CI 1.3 to 2.6), overweight (aOR 1.9 CI 1.4 to 2.4), and muscular pain and tension (aOR 1.8, CI 1.4 to 2.4), were all associated with reporting suicidal thoughts at follow-up. CONCLUSIONS: One in six young adults experienced suicidal thoughts, girls predominating. Suicidal thoughts were most strongly associated with symptoms of anxiety/depression, conduct problems, pain/tension and overweight reported when participants were 13-15 years old. Specific preventive efforts in these groups might be indicated. Future research should investigate whether similar associations are seen with suicide/suicidal attempts as endpoints.