To evaluate the safety and effectiveness of the long-term administration of diazoxide in patients with hyperinsulinemic hypoglycemia, a post-marketing surveillance study was conducted. Between 2008 and 2015, with a maximum observation period of 7 yr, 384 patients were monitored; 117 (30.5%) experienced at least one adverse drug reaction (ADR). The most commonly observed ADR was hypertrichosis (8.6%). The incidence of water retention-related ADRs and cardiac failure-related ADRs was 8.3% and 3.4%, respectively, and many of these occurred within the first 2 mo of treatment. The mean fasting blood glucose level was 44.9 mg/dL at baseline and was maintained at > 70 mg/dL, the control target, for 4 yr. A total of 113 infants < 1 yr of age were evaluated for the prognosis for intelligence, and a majority (77.9%) were assessed as “normal” at the final evaluation. Most ADRs occurred at an early stage of treatment and blood glucose levels were well controlled during long-term administration. The proportion of “normal” patients tended to be higher in those who started treatment at a younger age. However, because of the exploratory nature of this analysis, potential effects of coexisting or underlying diseases and the age of onset or diagnosis should not be ignored.