Renal function in 32 patients treated with lithium for an average period of 10 years was reexamined 2 years after the first examination. A markedly influenced tubular function leading to increased urine volume (average 3 litres/24 h) and decreased renal concentrating capacity was still found, whereas glomerular function remained unimpaired in nearly all of the patients. No statistically significant changes in renal functions were observed at the follow-up examination. The results were compared with the same renal functional tests obtained from a control group consisting of 53 patients with affective disorders never treated with lithium. The control group had a significantly lower urine output (average 2 litres/24 h), but lithium-treated patients on a one-dose schedule had an average urine volume of only 500 ml/24 h more than the controls. In conclusion, this prospective study found no evidence of a progressive impairment of glomerular or tubular function in lithium-treated patients reexamined after 2 years. Patients with affective disorders never treated with lithium had normal renal concentrating capacity.