Hospital records are used to identify suicide attempts in many countries but not all individuals present to hospital after a suicide attempt i.e., suggesting a ‘hidden number’. Our aim was to present the prevalence of suicide ideation, plans, attempts, and suicides among Danish adolescents, including attempts not resulting in hospital contact. The study population consisted of participants in the Danish National Birth Cohort participating in an 18-year follow-up, with individual-level linkage to national register data. Prevalence was estimated with a variable with mutually exclusive categories ranging from no suicidality to self-reported suicide ideation, -plans, -attempt and hospital-recorded suicide attempt and stratified on sex and parental income. The ‘hidden number’ was estimated as the ratio between suicide attempts with and without hospital contact. Among 47 858 participants, all aged 18-years, 36% girls and 28% boys reported suicide ideation at least once in their life. In addition, 6% girls and 3% boys had either reported or been recorded with a suicide attempt. For every attempt recorded in the hospital setting, two girls (ratio, 1:2) and six boys (ratio, 1:6) reported having attempted suicide without hospital contact. The prevalence of any suicide attempt was 8% and 3% in the lowest and highest income group, respectively. Before age 18, 0·011% girls and 0·016% boys had died by suicide. In conclusion, suicidal ideation and behaviour are common in adolescents and there is a substantial ‘hidden number’ of adolescents with suicide attempt. These results emphasize the need for early age suicide preventive interventions in community-settings e.g., school environments.