Objective: Bipolar disorder (BD) affects 1-4% of the population worldwide and presents with seasonal patterns with an incidence of 20-30%. Limited studies exist on seasonality, its relation to sociodemographic and clinical characteristics, and effects on treatment in BD in Turkey. Methods: This retrospective study included 174 patients with a major diagnosis of BD according to the DSM-IV. Comorbidities were determined using the SCID-I. The study was conducted using a semi-structured interview schedule and a mood monitoring chart. The patients were divided into two groups, those with and those without seasonal patterns, and their sociodemographic and clinical characteristics were compared. Results: We found that 36.2% of patients with BD had seasonal patterns. Regardless of type, first episodes were significantly more frequent in the non-seasonal group. With respect to first-episode types, the manic type was significantly more frequent in the seasonal group and the depressive type in the non-seasonal group. The number of patients with a comorbid anxiety disorder (NOS) or dysthymia was significantly higher in the seasonal group and the hospitalized non-seasonal group. The rate of response to lithium monotherapy, mean duration of the disease, mean total number of episodes, and mean total number of mixed episodes were significantly higher in the seasonal group. Conclusion: The rate of dysthymia was significantly higher in the seasonal group, suggesting that those with seasonality have more depressive aspects. These results indicate that the prognosis for seasonal bipolar patients is poorer than for non-seasonal patients. Another important finding was the presence of a correlation between seasonality and a positive response to lithium protection. It has been established that the first episode of the disorder was manifested in the seasonal pattern. The first episode of BD is known to often develop as a depressive episode. These results suggest that it may be a predictor of seasonality. [Anadolu Psikiyatri Derg 2017; 18(6.000): 571-576]