Background: A season of birth tendency has been shown for several psychiatric disorders and suicidal behavior. Our aim was to examine the association between season of birth and self-mutilative behavior (SMB) among adolescent psychiatric inpatients. Methods: The study sample consisted of 508 (40.9% males) 12- to 17-year-old adolescents consecutively admitted to the Department of Psychiatry of Oulu University Hospital, Finland, between April 2001 and May 2006. The birth month of each adolescent was categorized into one of the four seasons: spring (March-May), summer (June-August), autumn (September-November) or winter (December-February). The information on SMB was based on the K-SADS-PL interview, which included an item on nonsuicidal physical self-damaging acts without intent to die. A total of 144 adolescents (27 males, 117 females) met the criteria for SMB. The association between season of birth and SMB was assessed with a logistic regression analysis after controlling for each adolescent's age, previous suicide attempts and DSM-IV-diagnosed psychiatric disorders. Results: The monthly distribution of births of adolescents with SMB differed statistically significantly from that observed in the general population of the same age. An association between season of birth and SMB was seen in girls, but not in boys. The likelihood for SMB was significantly increased (adjusted OR 2.9; 95% CI 1.4-6.2) among girls born in autumn compared to those born in winter. Conclusions: Birth during autumn may predispose girls to SMB via dysfunctional serotonergic or other neurotransmitter systems. These findings may also be related to seasonal rhythms in parental mood and poor early care of the offspring. Copyright (c) 2007 S. Karger AG, Basel.