Clinicians commonly incorporate adolescents’ self-reported suicidal ideation into formulations regarding adolescents’ risk for suicide. Data are limited, however, regarding the extent to which adolescent boys’ and girls’ reports of suicidal ideation have clinically significant predictive validity in terms of subsequent suicidal behavior. This study examined psychiatrically hospitalized adolescent boys’ and girls’ self-reported suicidal ideation as a predictor of suicide attempts during the first year following hospitalization. A total of 354 adolescents (97 boys; 257 girls; ages 13–17 years) hospitalized for acute suicide risk were evaluated at the time of hospitalization as well as 3, 6, and 12 months later. Study measures included the Suicidal Ideation Questionnaire-Junior, Multidimensional Anxiety Scale for Children, Children’s Depression Rating Scale-Revised, Beck Hopelessness Scale, Youth Self-Report, and Personal Experiences Screen Questionnaire. The main study outcome was presence and number of suicide attempt(s) in the year after hospitalization, measured by the Diagnostic Interview Schedule for Children. Results indicated a significant interaction between suicidal ideation, assessed during first week of hospitalization, and gender for the prediction of subsequent suicide attempts. Suicidal ideation was a significant predictor of subsequent suicide attempts for girls, but not boys. Baseline history of multiple suicide attempts was a significant predictor of subsequent suicide attempts across genders. Results support the importance of empirically validating suicide risk assessment strategies separately for adolescent boys and girls. Among adolescent boys who have been hospitalized due to acute suicide risk, low levels of self-reported suicidal ideation may not be indicative of low risk for suicidal behavior following hospitalization.