OBJECTIVES: In a previous study about severe hypoglycemic coma events (SH) among diabetes mellitus (DM) patients we saw regional variation in SH occurrence. The purpose was to compare the incidence of SH between hospital districts in Finland. METHODS: This study among type 1 (T1) and 2 (T2) diabetic patients was based on nationwide health care registers. The study cohort contained insulin naïve diabetic patients who had filled at least one prescription of insulin during follow-up in 2006-2009. SH was defined as a hospitalization or a secondary health care visit due to hypoglycemic coma (ICD-10: E10.00 and E11.00). Stratified incidence rates and adjusted hazard ratio (HR) estimates with 95% confidence intervals (CI) were calculated. Analyses were performed for the first and recurrent SHs.  RESULTS: The population comprised 5271 (17.6%) patients with T1 and 24602 (82.4%) with T2. Altogether 3.1% patients experienced at least one SH during the follow-up. In different hospital districts the rate of first SHs varied from 5.6 (Varsinais-Suomi) to 47.8 (Länsi-Pohja) for T1, and from 4.8 (Pohjois-Karjala) to 50.3 (Länsi-Pohja) for T2. Compared to the capital area, the risks of first SHs varied differently for T1 and T2, but were commonly higher in Länsi-Pohja (T1: HR 3.46, CI 1.41-8.46; T2: HR 1.60, CI 1.13-2.29) and lower in Pohjois-Karjala (T1: no events; T2: HR 0.275, CI 0.14-0.56). Taking all SH events into account did not change these differences remarkably.  CONCLUSIONS: We found differences between regions in risk of hospital-treated SH in both DM types. Further analyses will be performed for e.g. ambulance density.