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Blood glucose control in patients with diabetes mellitus (DM) is reportedly influenced by the seasons, with hemoglobin A1c (HbA1c) levels decreasing in the summer or warm season and increasing in the winter or cold season. In addition, several studies have shown that sepsis is also associated with the seasons. Although both blood glucose control and sepsis can strongly affect the occurrence of severe hypoglycemia, few studies have examined the seasonal variation of severe hypoglycemia. The aim of the present study is to examine the association between severe hypoglycemia and the seasons in patients with type 1 diabetes mellitus (T1DM), type 2 diabetes mellitus (T2DM), and non-diabetes mellitus (non-DM). We retrospectively reviewed all the patients with severe hypoglycemia at a national center in Japan between April 1, 2006 and March 31, 2012. A total of 57,132 consecutive cases that had visited the emergency room by ambulance were screened, and 578 eligible cases of severe hypoglycemia were enrolled in this study. The primary outcome was to assess the seasonality of severe hypoglycemia. In the T1DM group (n = 88), severe hypoglycemia occurred significantly more often in the summer than in the winter (35.2% in summer vs 18.2% in winter, P = 0.01), and the HbA1c levels were highest in the winter and lowest in the summer (9.1% [7.6%–10.1%] in winter vs 7.7% [7.1%–8.3%] in summer, P = 0.13). In the non-DM group (n = 173), severe hypoglycemia occurred significantly more often in the winter than in the summer (30.6% in winter vs 19.6% in summer, P = 0.01), and sepsis as a complication occurred significantly more often in winter than in summer (24.5% in winter vs 5.9% in summer, P = 0.02). In the T2DM group (n = 317), the occurrence of severe hypoglycemia and the HbA1c levels did not differ significantly among the seasons. The occurrence of severe hypoglycemia might be seasonal and might fluctuate with temperature changes. Patients should be treated more carefully during the season in which severe hypoglycemia is more common.