BACKGROUND: Accumulating evidence has converged to suggest that childhood trauma may contribute to bipolar disorder (BD). This study aimed to investigate the patterns of childhood trauma among patients with bipolar I (BD-I) and bipolar II (BD-II) disorders, according to DSM-IV and in contrast with healthy volunteers. We also explored whether the relationship between childhood trauma and onset of bipolar disorder is mediated by immature defense mechanisms. METHODS: Participants were patients with BD-I (n=44) and BD-II (n = 42), and healthy controls (HCs, n = 43). Childhood traumatic experiences and defense mechanisms were assessed by the Childhood Trauma Questionnaire (CTQ) and the Defense Style Questionnaire (DSQ), respectively. RESULTS: BD patients experienced more severe childhood trauma than HCs. Physical neglect sub-score and total score of the CTQ had both direct and indirect effects on the diagnosis of BD-I, and an immature defense style mediated the indirect effects. The diagnosis of BD-II was mainly related to the physical neglect and emotional abuse subs-core and total score of the CTQ, as mediated by the immature defense mechanisms. BD-I and BD-II significantly differed in the emotional abuse sub-score of the CTQ. CONCLUSIONS: Physical neglect sub-score and total score of the CTQ were associated with the diagnosis of BD (both BD-I and BD-II), as mediated by an immature defense style. Furthermore, emotional abuse might be an important risk factor for BD-II compared to BD-I. These findings may inform risk reduction and psychosocial intervention strategies to prevent and treat patients with bipolar disorders.