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Purpose: Social isolation is associated with suicidal ideation (SI) and self-harm (SH) among adolescents. However, the association between preference for solitude (PfS), SI, and SH is unknown. The prevalence of adolescents who have both of PfS and social isolation and the risks for SI and SH among them are also unknown. Methods: Information on PfS, social isolation, SI, and SH was collected in a large-scale school-based survey on adolescents, using a self-report questionnaire. Associations between PfS, SI, and SH were examined by logistic regression analysis. The interactions between PfS and social isolation on SI and SH were also investigated. The odds of SI and SH were examined for groups defined by presence of PfS and social isolation. Results: Responses from 17,437 students (89.3% of relevant classes) were available. After adjusting for demographic characteristics and social isolation, PfS was associated with increased odds of SI (odds ratio [OR] 1⁄4 3.1) and SH (OR 1⁄4 1.9). There was no interaction between PfS and social isolation on SI and SH. After adjusting for demographic characteristics, the odds for SI (OR 1⁄4 8.6) and SH (OR 1⁄4 3.8) were highest among adolescents with both PfS and social isolation (8.4% of all respondents). Conflicts of Interest: The authors have no conflicts of interest to disclose. Clinical trials registry site and number: none. * Address correspondence to: Shinji Shimodera, M.D., Ph.D., Department of Neuropsychiatry, Kochi Medical School, Kochi Uni Kochi 783-8505, Japan. E-mail address: shimodes@kochi-u.ac. jp (S. Shimodera). 1 Equal contribution to first author. 1054-139X/ 2017 Society for Adolescent Health and Medicine. Published by Elsevier Inc. This is an open access article un creativecommons.org/licenses/by-nc-nd/4.0/). http://dx.doi. org/10.1016/j.jadohealth. 2017.02. 018 CONTRIBUTION Adolescents with a preference for solitude, particularly thosewho are socially isolated, are at increased risk of suicidal ideation and self-harm. These results suggest focusing suicide prevention efforts on those who say they prefer solitude and have no one to consult with about worries or troubles. versity, Oko-cho Kohasu, Nankoku-shi, der the CC BY-NC-ND license (http:// K. Endo et al. / Journal of Adolescent Health xxx (2017) 1e5 2 Conclusions: PfS was associated with increased odds of SI and SH in adolescents. No interaction effect between PfS and social isolation on SI and SH was found, but adolescents with PfS and social isolation had the highest risk for SI and SH. Parents and professionals should pay attention to suicide risk in adolescents with PfS. 2017 Society for Adolescent Health and Medicine. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Suicide is the second leading cause of death in young people, accounting for 8.5% of total deaths in young people [1]. In Japan, the leading cause of death among 15to 39-year-olds is suicide, while road traffic accidents is the leading cause in many other developed countries [2]. Also, self-inflicted injury is one of the leading causes of disability-adjusted life years in young people [3]. Social isolation has been identified as a risk factor for suicidal problems [1,4]. Since a rapidly changing social world increases the likelihood of social isolation during adolescence [5], more attention should be paid for adolescents who experience social isolation. Preference for solitude (PfS) in adolescence can be regarded as a developmental process. PfS has been said as a broad construct of unsociability and avoidance [6] and motivated by low approach and low-to-high avoidance [7]. The desire for solitude increases during adolescence, and time spent alone in adolescence can sometimes be beneficial for adjustment, perhaps because solitude facilitates individuation or identity formation [8]. As children grow older, solitude becomes more acceptable and they report solitude as more positive and important [8e10]. It has also been suggested that spending time alone in a volitional and autonomous manner is associated with higher levels of well-being [11]. On the other hand, negative aspects of PfS have also been reported. Several studies showed that PfS in early adolescence is associated with peer difficulties and maltreatment [12e14] and, in adolescence more generally, with low self-esteem, anxiety/depression, and emotional dysregulation [6,7]. Therefore, it is possible that risk for suicidal ideation (SI) and self-harm (SH) is increased in adolescents with PfS. However, to the best of our knowledge, this has not been investigated. Furthermore, no studies have examined the prevalence of adolescents who had both PfS and social isolation and the risk of SI and SH among them. The objective of this study therefore was to examine these possible associations.