Objective: In literature, theres data about how the season of birth of bipolar disorder patients is important to determine the etiology and the course of the disorder. In this study, its intended to find out the relationship between season of birth and clinical characteristics of patients in a mood disorders clinic. Methods: Three hundred and seventy-five cases out of 1600 patients from Prof.Dr. Mazhar Osman Mental Health and Neurological Diseases Education and Research Hospital were included in the study. Using the data form created by the researchers, birth months, social and demographical characteristics, age of onset of the disorder, clinical course and family history data were collected. Results: Two hundred and twenty-eight (60.8%) patients were female. The diagnosis of 363 (96.8%) patients were BP (bipolar disorder) I, 12 (3.2%) patients were BP II. When grouping according to the seasons; birth month of 102 (27.2%) patients were winter months, 108 (28.8%) patients were born in spring, 91 (24.3%) patients during the summer months, and 74 (19.7%) patients were born in the autumn. There was no statistically significant relationship between season/month of birth and type of first-episode, positive family history of psychiatric disorder. Statistically significant relationship was found between age of onset and season of birth. Conclusion: To conclude the birth rates of winter and spring season is higher than summer and autumn season. Because a control group was not included in the study, statistical significance could not be revealed. There was no statistical significance between season of birth and course features and clinical characteristics. [Anadolu Psikiyatri Derg 2014; 15(4.000): 313-317]