Objectives To evaluate the incidence and outcomes of self-harm from ambulance records.

Design A retrospective, observational study.

Setting Osaka City, Japan.

Participants A total of 365 adolescents aged 10–19 years with emergency self-harm such as poisoning by drugs or gas, cutting skin, jumping from heights, hanging and drowning and treated by emergency medical service personnel from January 2010 through December 2012.

Primary outcome measurements Incidence per 100 000 persons and outcome at the scene or hospital arrival by age and gender. Poisson regression models for incidence evaluation were used; reporting relative risks (RRs) and their 95% CIs.

Results During the study period, a total of 425 self-harm events were documented in 365 adolescents. The incidence of self-harm increased significantly between the ages of 11 and 19 years, from 6.3 to 81.0 among boys and the ages of 12 and 19 years from 6.3 to 228.3 among girls, respectively (both p<0.001). Although there was no incidence difference between girls and boys in the group aged 11–14 years (RR 1.20; 95% CI 0.59 to 2.47), the incidence was significantly higher among girls than boys in the group aged 15–19 years (RR 4.18; 95% CI 3.20 to 5.45). The overall proportion of death by self-harm was 4.9%. The proportion of hospital admission and death by self-harm was higher among boys than among girls (38.6% vs 25.2%, p=0.016 and 14.8% vs 2.4%, p<0.001).

Conclusions The incidence of emergency treatment for self-harm by adolescents increased with age and our findings also demonstrated the gender paradox. It would be necessary to establish active, gender-specific and comprehensive prevention strategies for adolescent self-harm, based on our findings showing the age and gender differences of self-harm among adolescents.