Although the study of medically serious suicide behavior is an important strategy for understanding the nature of suicide, little is known about its underlying psychological mechanisms. This gap is addressed here by applying insights from attachment theory to severe suicidal behavior. The results show that both anxious and avoidant attachment patterns predict medical lethality. Path analysis indicated that interpersonal difficulties mediated the paths between insecure attachment patterns and lethality of suicide attempts. These results suggest that the psychological mechanisms of medically serious suicide behavior involve high levels of mental pain amplified by insecure attachment patterns and interpersonal difficulties. Implications for prevention and therapeutic intervention strategies are discussed.