Nephrogenic diabetes insipidus usually presents with polyuria, polydipsia, fever, vomiting, dehydration and failure to thrive. However, in infancy polyuria may be absent because of dehydration and reduced glomerular filtration rate. In 2 cases the main presenting feature was hypotonia, with marked head lag. Family studies confirmed the X-linked mode of inheritance of the disease; in case 1 the disease appeared to have arisen as a new mutation in the mother, and in case 2 the carrier status was traced back to the great-grandmother. Pitfalls in the diagnosis and detection of the carriers are discussed. Treatment with thiazide diuretics and prostaglandin synthesis inhibitors is effective in reducing urine volumes and polydipsia. The early detection of the disease and adequate management may prevent such complications as megacystis, mega-ureter and hydronephrosis, with resulting renal failure. Mental and physical retardation may also be avoided.