In our hospital, we are studying the renal function of more than 200 patients under lithium treatment. The glomerular function seems to be affected to a small extent, even by long-term lithium administration, and there have been no instances of progressive deterioration of renal function with azotemia. Most of the patients have impaired water reabsorption with polyuria and polydipsia. This is frequently combined with lowered concentrating ability. Polyuria and lowered concentrating ability are not in themselves life-threatening. However, development of dehydration may lead to lithium poisoning, and patients under lithium treatment should be advised to drink extra fluid under circumstances when the fluid balance may become negative. Further studies are required for definitive recommendations concerning extra monitoring of kidney function and lowering of lithium doses.