Abstract Background Substantial numbers of adolescents self-harm, but the majority of cases do not reach the attention of medical services, making community studies essential. The prevalence of suicidal thoughts and plans at this age, and the inter-relationships between suicidal thoughts, plans and self-harm remain largely unexplored. Method Cross-sectional analysis of self-reported questionnaire data collected from members of the Avon Longitudinal Study of Parents and Children (ALSPAC) birth cohort, England. Respondents (n = 4810) were aged 16–17 years old and have been followed up since birth. Results Altogether 905 (18.8%) respondents had ever self-harmed. The prevalence of lifetime self-harm was higher in females (25.6%) than males (9.1%). The most commonly used method was self-cutting: this was used alone or in combination in 73.5% of episodes, compared to 10.0% who took overdoses alone or in combination with other methods. Of those who reported self-harm, 25.3% wanted to die during the most recent episode. Concurrent depression was associated with a greatly increased risk of self-harm (OR 5.43). Only 12.4% of participants sought medical help following their most recent episode of self-harm, although this figure was higher (30.1%) where self-harm was carried out with desire to die. Of the whole sample, 15.8% had ever thought of killing themselves, and 4.3% had ever made plans to kill themselves. Compared to those who had never self-harmed, those who had self-harmed but not wanted to die during the most recent episode were at increased risk of ever having had suicidal thoughts (37.6% compared to 7.8% χ 2 =102.3, p