Hyperglycaemic hyperosmolar syndrome is a major acute complication of decompensated diabetes mellitus. It represents the second most common aetiology of diabetic coma and is associated with excess mortality. It is characterised by severe hyperglycaemia, hyperosmolality and dehydration in the absence of significant ketosis, afflicting principally middle-aged-to-elderly patients. Early clinical diagnosis and prompt treatment, consisting of fluid replacement, insulin therapy, restoration of electrolyte disturbances and management of concurrent illnesses may improve the outcome. This review provides an outline of the diagnostic approach of patients with manifestations of hyperglycaemic hyperosmolar syndrome and discusses the contemporary therapeutic recommendations.