Correlations between quantitative kidney biopsy findings and clinical renal function in 46 unselected patients treated with lithium for an average of eight years were studied. A significant relationship between maximum renal concentrating capacity and degree of tubular atrophy was found. GFR correlated significantly with sclerotic glomeruli as well as atrophic tubules in patients on a multiple dosage schedule, whereas no relationship was seen in patients receiving lithium in a single daily dose. Thus, renal dysfunction may have a structural basis in a subgroup of lithium-treated patients on a multiple dosage schedule.