The function of the kidney in adult subjects with Cooley's disease has been hitherto inadequately investigated. In 10 patients suffering from thalassemia major, glomerular filtration rate, renal plasma flow, renal blood flow have been studied, together with urine concentration capacity. The authors emphasize certain pathological changes, such as the increase in renal plasma flow and the failure of the urine concentration ability. On the basis of renal biopsies, two pathogenetic theories are discussed: the functional impairment of the coutercurrent system and fibrosis of the medullary tissue.