Neonatal diabetes mellitus (NDM) is a very rare disorder occurring between 1:400 000 and 1:500 000 live births and, until recently, little was known about this disease. The authors report a case of transient NDM in a 2-day-old female infant admitted in an intensive care unit with a blood glucose level greater than 400 mg/dl, glycosuria, ketonuria and with no evidence of autoimmunity. Treatment with insulin was necessary until the 51st day of life and many difficulties were found in the management of metabolic control because of the need for tiny quantities of insulin. Hyperglycaemia is sometimes routinely treated with insulin by neonatologists but after excluding the common causes of hyperglycaemia, NDM should be considered as a diagnostic possibility with clinical, genetic and therapeutic implications.