INTRODUCTION: The objective of the present study was to investigate the defense style of insomnia patients and to grasp the differences in defense style between primary insomnia patients and insomnia patients with history of major depressive disorder. METHODS: Forty three subjects with insomnia (11 subjects with primary insomnia and 32 subjects with major depressive disorder) and 138 control subjects participated in this study. To diagnose insomnia and major depressive disorder, interviews including structured clinical interview for DSM-IV (SCID-IV) were done. To assess the defense style, self-reported Korean version of Defense Style Questionnaire (K-DSQ) were completed by the participants. RESULTS: Compared to normal controls, subjects with insomnia used more acting out (t=3.25, p<0.01), consumption (t=2.66, p<0.01), fantasy (t=3.51, p<0.001), resignation (t=5.42, p<0.001), suppression (t=3.28, p<0.01), projection (t=3.92, p< 0.01), splitting (t=4.31, p<0.01), undoing (t=2.66, p<0.01), withdrawal (t=6.72, p<0.001) and isolation (t=3.80, p<0.001), and less omnipotence (t=4.08, p<0.001) and humor (t=3.20, p<0.01). Compared to normal controls, subjects with primary insomnia used more undoing and withdrawal. Compared to subjects with primary insomnia, subjects with insomnia with history of major depressive disorder used more resignation and withdrawal, and less humor. CONCLUSION: In the current study, there were differences in defenses between primary insomnia patients and insomnia patients with major depressive disorder history. To evaluate the pattern of defenses through the K-DSQ might provide important clues to differentiate these two conditions.