Renal function and structure were studied in 57 patients on long-term lithium treatment. Nine per cent had a moderate decrease in glomerular filtration rate (GFR), 75% had decreased concentrating ability. No correlation between GFR and the duration of the lithium treatment was found. Twenty-four renal biopsy specimens were studied by light microscopy and compared with kidney biopsies from an age-matched control group of patients without arterial hypertension and who had never been treated with lithium or neuroleptics, but who suffered from acute oliguria or slight proteinuria. The number of sclerotic glomeruli and the focal distribution of intestitial fibrosis was significantly increased in the group of patients treated with lithium. No difference in the total amount of fibrous tissue or the number of atrophic tubules was demonstrated. There was no correlation between the duration of lithium treatment and the structural changes. The biopsy material originated from patients treated with a single and reduced dose regimen only. This regimen might explain the few and slight structural changes found, in contrast to other investigations in which patients were treated with lithium in divided doses.