Background: Clinical and research findings have highlighted the role of interpersonal factors in suicidal behavior with high levels of intent and lethality. Schizoid personality disorder (SPD) is at the extreme end of interpersonal difficulties. Thus, we aimed to understand the contribution of SPD symptoms to suicide behavior and specifically to more lethal suicide attempts. Method: Four groups were investigated (N = 338): medically serious suicide attempters, medically non-serious suicide attempters, psychiatric and healthy controls. SPD symptoms, mental pain variants, and clinical characteristics were assessed. Results: Overall, attempters were characterized by higher levels of most SPD symptoms. Solitary lifestyle and emotional detachment were higher among medically serious suicide attempters relative to less-serious attempters. Emotional detachment doubled the risk for high lethality, beyond mental pain variables. Conclusions: SPD symptoms of interpersonal difficulties and low levels of emotional expressions are important risk factors for more severe suicidal behavior. Implications for identification of at-risk groups for suicide are discussed.