Kidney function has been examined in 237 patients who in the autumn of 1977 were in lithium treatment at the Psychiatric Hospital in Risskov, most of them as outpatients. The average age was 42 years. The patients had been given lithium treatment for 0.5-17 years, mean duration 5 years. The mean lithium dosage was 33 mmol/day and the mean 12-hour serum lithium concentration 0.85 mmol/l. Glomerular filtration rate was assessed through determination of 24-hour creatinine clearance and serum creatinine, in some cases iothalamate clearance. Water excretion was assessed through determination of 24-hour urine volume and in some cases urine osmolality after 26 hours of fluid deprivation. Creatinine clearances, serum creatinine concentrations, and urine volumes were subjected to multiple regression analysis with various clinically relevant predictor variables. Affection of glomerular filtration rate was only moderate and progressed slowly. The data indicate that the risk of renal insufficiency and terminal azotemia is remote even when lithium is given for many years. A large number of the patients had altered water excretion with polyuria or lowered urine concentrating ability or both. Due to the extra fluid loss these patients are apt to develop dehydration, and they may then be in danger of lithium poisoning. We hypothesize that lithium-induced changes of kidney function may become less frequent and less pronounced if patients are maintained at serum lithium levels somewhat lower than those employed in the group studied here. We recommend careful monitoring of serum lithium levels, regular control of kidney function, and extra caution when physical illness or additional drug treatment may lead to disturbance of fluid and electrolyte balance.