Background: Relatively few studies test the interpersonal psychological theory's monotonicity hypothesis. The monotonicity hypothesis proposes that suicide capability (i.e., fearlessness about death and pain tolerance) is stable or increases linearly with exposure to painful and provocative events. Research is conflicted, suggesting that suicide capability is static, decreases, or increases and returns to baseline. The current study thus tested this hypothesis in a sample of college students with histories of suicidal ideation. We hypothesized a stable and an increasing trajectory. Methods: Participants were 206 undergraduates; primarily women (73%), on average 19.05 years old, heterosexual (85%), and first-years (69%). Participants completed a baseline battery of questionnaires on suicide risk factors and daily diaries on suicide capability and suicidal ideation for 90 days (n = 7,342 surveys, 40% compliance rate). Group-based trajectory analyses were conducted with the SAS macro PROC TRAJ. Results: Modeling revealed a three group quadratic model. Low (27.7%), Moderate (41.3%), and High (31.1%) suicide capability groups remained static over time. Baseline suicidal ideation, but not history of suicide attempts or family history of suicidal behavior, distinguished groups; participants with suicidal ideation at baseline were less likely to be in the low suicide capability group. Limitations: Brief, dichotomized assessments, and a high attrition rate. Conclusions: These data showed temporal stability of suicide capability and suggest that the “acquired” component of capability may be overemphasized. Clarifying the stability and modifiability of suicide capability will enable empirically-based applications of the theory to suicide prevention.