Theoretical and empirical literature suggests that nonsuicidal self-injury (NSSI) may represent a particularly important risk factor for suicide. The present study examined the associations of NSSI and established suicide risk factors to attempted suicide in four samples: adolescent psychiatric patients (n = 139), adolescent high school students (n = 426), university undergraduates (n = 1,364), and a random-digit dialing sample of United States adults (n = 438). All samples were administered measures of NSSI, suicide ideation, and suicide attempts; the first three samples were also administered measures of depression, anxiety, impulsivity, and borderline personality disorder (BPD). In all four samples, NSSI exhibited a robust relationship to attempted suicide (median Phi = .36). Only suicide ideation exhibited a stronger relationship to attempted suicide (median Phi = .47), whereas associations were smaller for BPD (median rpb = .29), depression (median rpb = .24), anxiety (median rpb = .16), and impulsivity (median rpb = .11). When these known suicide risk factors and NSSI were simultaneously entered into logistic regression analyses, only NSSI and suicide ideation maintained significant associations with attempted suicide. Results suggest that NSSI is an especially important risk factor for suicide. Findings are interpreted in the context of Joiner's interpersonal-psychological theory of suicide; specifically, NSSI may be a uniquely important risk factor for suicide because its presence is associated with both increased desire and capability for suicide.