A case of Rabson—Mendenhall syndrome in a boy is described. Rabson—Mendenhall syndrome is an extremely rare form of insulin resistance and is characterized by a severe course. To date, less than 50 cases of the syndrome have been reported around the world. Its prevalence is unknown because cases ending in death at early age often remain undiagnosed. The most pathognomonic features include acanthosis nigricans and hyperkeratosis of the skin in the area of natural folds, which reflect insulin resistance. Diabetes mellitus in children with Rabson—Mendenhall syndrome develops in childhood and has a labile course, which is manifested by frequent ketoacidosis episodes. Severe insulin resistance in patients with Rabson—Mendenhall syndrome prevents compensation of diabetes mellitus. Chronic glucosotoxicity in these children leads to progression of diabetic complications and to early disability. The literature reports single cases of long-term follow-up of patients with Rabson—Mendenhall syndrome.