Nonketotic hyperosmolar diabetic coma is a rare manifestation of juvenile diabetes, in contrast to adult onset diabetes. To date only 20 cases have been published, the majority of them infants and toddlers. This type of diabetic coma is seen with unusual frequency in children with Down's syndrome and psychomotor retardation. The clinical picture is characterised by severe dehydration, hyperglycemia with often extremely high blood sugar levels, hyperosmolarity and glucosuria without ketonuria. Mortality in children has been high (24%). This paper reports the case of a 14-month-old girl with Down's syndrome. Clinical and therapeutic as well as pathogenetic aspects are discussed.