One-hundred and ten patients treated with lithium for more than six months were studied in order to determine the prevalence of lithium induced nephropathy. Eighteen of 69 patients (26 per cent) who had been treated for more than two years presented a chronic interstitial nephropathy characterized by a marked decrease in renal concentrating ability with a disproportionate preservation of glomerular filtration rate. Histologically, increased amounts of fibrotic tissue in the medulla and the cortex were found together with tubular atrophy. In 40 per cent of the patients who underwent renal biopsy, cystic formations in the cortex were found. The impairment of renal concentrating ability could be related to the duration of lithium treatment and the degree of tubular damage correlated with the degree of impairment of renal concentrating ability. Lithium induced, chronic nephropathy is a rather common complication of long-term lithium treatment and reduces the patients capacity to regulate water and electrolyte metabolism. As water and electrolyte loss appears to precede the slowly progressing lithium intoxication, the main hazard of lithium induced nephropathy is lithium intoxication.