Disorders of fluid and electrolyte metabolism in elderly diabetics were studied. High frequency of hyperkalemia (20.8%), hypomagnesemia (14.6%), hypocalcemia (13.7%), hyperphosphatemia (8.6%), hyponatremia (8.1%) and hyperchloremia (7.2%) was observed among 332 elderly diabetics. Furthermore, hyperkalemia, hyperphosphatemia, hyponatremia, hyperchloremia, hypercalcemia and hypermagnesemia were more frequent in diabetics with renal insufficiency (serum Cr greater than or equal to 1.5 mg/dl) than in diabetics with normal renal function (serum Cr less than or equal to 1.4 mg/dl). In addition, statistically significant negative correlation were observed between plasma glucose levels and serum levels of sodium and chloride in diabetics with normal renal function. These results clearly demonstrated that the most important causal factor of electrolyte disorders in elderly diabetics might be the renal dysfunction due to diabetic nephropathy and/or nephrosclerosis. Moreover, glucose intolerance is also one of the causal factors for hyponatremia and hypochloremia. Disorders of fluid and electrolyte metabolism were manifest in 31 diabetic patients with hyperosmolar non-ketotic coma. The frequency of patients with abnormally elevated serum levels of sodium, potassium and chloride, and patients with abnormally lowered serum levels of calcium was high in this morbid state. Water and sodium deficit, examined in 11 cases of hyperosmolar non-ketotic coma, was 4780 +/- 2100 ml (107 +/- 43 ml/kg body weight) and 290 +/- 170 mEq (6.8 +/- 4.2 mEq/kg body weight), respectively. However, no significant deficit of potassium was observed in the patients. Statistically significant positive correlations between water deficit and serum Cr levels and with serum effective osmolarity were observed. However, there were no significant correlations between water deficit and plasma glucose levels, serum sodium levels and serum osmolarity.(ABSTRACT TRUNCATED AT 250 WORDS)