The renal risks associated with long-term lithium treatment are a growing concern. We have therefore studied renal function by means of glomerular filtration rate (GFR) and maximum urinary concentrating capacity (Umax) in 142 of 215 patients with more than 15 years of lithium treatment in nine psychiatric clinics. Data on psychiatric and somatic diseases, hospital admissions, cumulative lithium doses, and other psychotropic treatments were extracted from the medical records. The patients were investigated according to a standardized protocol. GFR was measured as 51Cr EDTA clearance and Umax using the DDAVP test. Thirteen patients had had signs of lithium intoxication. GFR was reduced in 21% of the patients and Umax in 44%. Nephrogenic diabetes insipidus was present in 12%. Umax but not GFR was inversely correlated to the cumulative lithium dose. Kidney function was more reduced in patients on lithium combined with psychotropic treatment and/or concomitant treatment for somatic disorders. Thirst was a complaint of 53% of the patients, predominantly those with additional psychotropics. We conclude that kidney damage is common in patients on long-term lithium treatment and that both glomerular and tubular function are affected.