Albumin and betaa-microglobulin excretions were studied in 85 unselected patients treated with lithium for more than 6 months. In 15 patients a lithium-induced impaired renal concentrating ability was found, 13 of these patients revealed a chronic interstitial nephropathy on renal biopsy. There was no significant increase in albumin excretion, and a markedly increased beta2-microglobulin excretion was seen in only 1 patient. This study supports the hypothesis that the lithium-induced renal lesion is confined to the distal part of the nephron.