Background: The number of suicides in Japan has continued to exceed 30,000 every year, with the rate remaining at around 25.0 per 100,000 individuals since 1998.Among adolescents, suicide is the leading cause of death, ranking first as the cause of death among individuals aged 15-19.In this study, we examined the gender differences in adolescent suicide attempters in Japan.Methods: Participants were adolescent patients 18 years or younger treated at Nippon Medical School Hospital Critical Care Medical Centre, Tokyo, for suicide attempts between January 2006 and December 2010.A total of 24 adolescents were recognized as suicide attempters, 7 males (29%) and 17 females (71%), age range 13 to 18 years, mean 16.1 years, and standard deviation (SD) 1.5 years.Psychiatrists used DSM-Ⅳ-TR criteria for the diagnoses.Results: At the initial assessment, males and females did not differ in age (males, 16.4 years, SD: 1.5; females, 16.0 years, SD: 1.6 years).For males, the most frequent diagnosis, with repetitions, was schizophrenia (n=4, 57%), followed by pervasive developmental disorders (n=3, 43%), and adjustment disorders (n=1, 14%).For females, the most frequent diagnosis, with repetitions, was personality disorders (n=8, 47%), followed by mood disorders (n=5, 29%), substance-related disorders (n=1, 6%), schizophrenia (n=1, 6%), adjustment disorders (n=1, 6%), anxiety disorders (n=1, 6%), pervasive developmental disorders (n=1, 6%), mental retardation (n=1, 6%), eating disorders (n=1, 6%), nothing (n=1, 6%).Statistical analysis by chi-square test showed significantly more cases of schizophrenia in males than in females (p<0.05).As for socio-demographic factors, females more often had a history of deliberate self-harm than males (p<0.05).However, they did not differ in terms of other socio-demographic factors (previous psychiatric history, housemate, fatherless or motherless family, previous familial psychiatric history, bullying or abuse), method (poisoning, jumping from high place, cutting), and motive (family relations, health issues, financial situation, work environment, male-female relationships, school environment, other unclear reasons) of suicide attempt.Conclusion: We found statistically significant gender differences in terms of psychiatric diagnosis and previous deliberate self-harm.These findings indicate that gender differences should be taken into account when dealing with adolescent suicide attempts.