BACKGROUND: Congenital hyperinsulinism (CHI) is a severe disease with a high risk of complications including neurological deficit. Persistent hypoglycemia in patients with focal form of CHI can not be managed with medical treatment in 96.4% of cases, what subsequently leads to surgical treatment. Currently, there is a lack of information regarding patients with focal form of CHI. This study is aimed at finding better approaches for diagnosis and treatment of patients with focal form of CHI. AIMS: To study clinical, genetic and PET/CT findings of the focal form of CHI in Russian group of patients. MATERIALS AND METHODS: The observational research included all patients with a histologically confirmed focal form of CHI, who were admitted to Endocrinology Research Centre during the period from January 2008 to January 2019. A statistical analysis of clinical data, genotype, and positron emission tomography (PET) with 18F-dihydroxyphenylalanine (18F-DOPA) was performed. The median follow-up was 18 months. RESULTS: The study included 31 patients with focal CHI (14 boys, 45.2%). All patients had a neonatal presentation of the disease and demanded high levels of continuous glucose infusion to maintain euglycemia. The difference between the age of hypoglycemia presentation and the age of diagnosis ranged from 1 day to 3.9 months. In all cases, diazoxide was found to be ineffective. However, in 9 patients, it was possible to withdraw continuous glucose infusion and maintain euglycemia using octreotide in the preoperative period. A molecular genetic study allowed us to detect diverse pathogenic variants in ABCC8 and KCNJ11 genes in 30 patients. According to PET data with 18F-DOPA, the pancreatic index (PI) varied widely from 1.16 to 3.59. After partial resection of the pancreatic region with insulin hypersecretion, all patients showed complete recovery. CONCLUSIONS: The focal form of CHI is a severe condition with high prevalence of neurological complications. For preoperative diagnosis of the morphological form of the disease, it is necessary to conduct genetic analysis and radionuclide studies. Solely evaluation of mathematical parameters in 18F-DOPA PET without taking into account the visual data and the results of genetic analysis does not allow establishing the robust diagnosis. Timely diagnosis, identification of risk factors, and prevention of complications of persistent hypoglycemia are important tasks for clinicians.