Few studies have longitudinally examined suicidal ideation in those with adolescent-onset borderline personality disorder (BPD). The current study aimed to examine the trajectory of suicidal ideation in adolescents with BPD longitudinally over 6 months, with follow-ups at 2, 4, and 6 months posthospitalization for elevated suicide risk. Results indicated that the BPD group exhibited a greater decrease in suicidal ideation in the months following hospitalization than those without a BPD diagnosis. The findings of this study indicated that suicidal ideation in adolescents with BPD is not stable, and although ideation may decrease quickly after hospitalization, regular assessment of ideation is recommended.