We conducted a longitudinal study from 2007-2015 to identify trends in cases of severe hypoglycemia at a Japanese diabetes outpatient clinic using the Computerized Diabetes Care-Metabolic Syndrome (CoDiC-MS^®) database. There were 178 severe hypoglycemic events in 70 patients during the 9-year study period (0.8 events per 100 person-year). Thirty-seven patients (52.8 %) experienced more than 2 events. Among the 37 patients with type 1 diabetes, 13 (35.1 %) had hypoglycemia unawareness. In the 33 who had type 2 diabetes, an older age, treatment with insulin or sulfonylureas, macrovascular complications and dementia were identified as potential predictors of severe hypoglycemia. Seventy-three events (41.0 %) were caused by daily living changes in the diet or activity habits, and 41 events (23.0 %) were of unknown cause. Our results suggest that preventive behavioral approaches are necessary for high-risk patients, especially for patients with hypoglycemia unawareness.