Rhabdomyolysis has been reported to be associated with hyperosmolality in diabetic ketoacidosis and non-ketotic hyperosmolal state. Whether the rhabdomyolysis was due to hyperosmolality per se or whether hyperglycaemia also played a role is not clear. We hereby report a case of cranial diabetes insipidus with hypernatraemia and hyperosmolality complicated by rhabdomyolysis. None of the known risk factors, such as coma, hypokalaemia, hypophosphataemia, diabetic ketoacidosis or non-ketotic hyperosmolality, were present in this patient. We believe that severe hyperosmolality per se is an important predisposing factor for non-traumatic rhabdomyolysis, and serum muscle enzymes should be closely monitored in the management of patients with diabetes insipidus.