Renal lithium clearance was investigated in 44 patients treated with lithium for an average of 8 years as part of a functional-morphological follow-up study including a kidney biopsy. The average renal lithium clearance was 0.36 ml/s (= 21.6 ml/min). A significant correlation with age, sex and glomerular filtration rate was seen, whereas no significant relationship with urine volume, lithium treatment regimen and histopathological biopsy variables was found. The results were compared with the same renal functional tests obtained from a control group consisting of 26 patients with affective disorders never treated with lithium. The control group had a lower urine output, but no significant difference in lithium clearance was observed. In conclusion, renal lithium clearance is a specific investigation, which may provide valuable baseline information on glomerulo-tubular function in patients before and during prophylactic lithium treatment.