Background: Suicidal behavior has been reported to be associated with seasonality and chronotype. However, no study has reported the suicidality according to chronotype and seasonality in patients with major depressive disorder (MOD). This study evaluated the relationship of suicidality and biological rhythm in patients with MOD. Methods: A total of 120 patients with NIDD participated. We evaluated their seasonal patterns, chronotypes (morningness, wake up early and are at their best during the first part of the day; eveningness, feel best and most alert during late hours), suicidal ideations, and other clinical variables including symptom severity. Results.: Patients with seasonality showed significantly higher suicidal ideation than patients without seasonality. Compared to morning-type participants, the evening-type participants showed higher suicidality. Suicidal ideation was positively correlated with eveningness tendencies and hypomanic personality traits. Limitations: First, we did not analyze the causal relationship between biological rhythm and suicidality. Secondly, suicidal ideation does not in most cases reflect an actual attempt. Lastly, we did not categorize seasonal pattern into summer or winter type. Conclusions: Our findings suggest that biological rhythm is significantly associated with suicidal ideation in patients with MDD. (C) 2014 Elsevier By. All rights reserved,