Seventeen unselected, consecutive patients with intracranial disease and accompanying hyponatraemia were studied. All would previously have been diagnosed as having the syndrome of inappropriate antidiuretic hormone (ADH) secretion on the basis of spot plasma/urinary electrolyte testing with the application to them of existing standard laboratory criteria. Timed urinary collections and matching plasma samples were available in all but three cases for the derivation of creatinine, osmotic and free-water clearances, tubular reabsorbed water, and fractional water and sodium excretions. In a number of patients the plasma renin, aldosterone and ADH levels were also assayed. On the basis of the overall findings, 13 patients were diagnosed as in fact having a salt-wasting state whilst in only four patients was the diagnosis of inappropriate ADH secretion (SIADH) substantiated. It is suggested that obtaining simple derived parameters of sodium and water homeostasis can add significantly in differentiating between these quite opposite syndromes.