To estimate the frequency of diuretic-related electrolyte disorders in the elderly, 561 consecutive admissions to three acute geriatric units were studied. For the 287 admissions to one unit, discharge/death diagnoses were also examined in relation to admission diuretic therapy. Sodium concentrations were significantly lower, and urea and creatinine significantly higher, in patients on diuretics, though the size of the differences was small. Comparing different preparations sodium concentrations were significantly lower on Moduretic than on Dyazide or Navidrex K and on frusemide when combined with a potassium-retaining diuretic rather than a potassium supplement. Potassium concentrations were significantly lower on Bendrofluazide alone compared to Navidrex K or Moduretic. Diuretics were positively associated with cardiac failure, ischaemic heart disease, airflow obstruction and obstructive large bowel disorders but negatively with Parkinson's disease. No significant association was found with falls, immobility or confusion. Major electrolyte disorders on diuretics appear to be unusual but important differences exist between preparations. Similarly major illness resulting from diuretic therapy is rare but minor morbidity may be more common.