The proportions of CD4+, CD8+ cells were determined in peripheral blood samples obtained from 27 patients with idiopathic haemochromatosis and 67 healthy controls. Samples from the patients were obtained at the start of therapy and at intervals during intensive phlebotomy treatment in which 400 ml of blood are removed each week for 6 months to 2 years. Patients varied in their CD4/CD8 ratios from 0.8 to 4.2. An individual's CD4/CD8 ratio, however, remained unaffected by the repeated removal of large volumes of blood by phlebotomy confirming the existence of a homeostatic regulation of the relative numbers of the two major peripheral T cell pools in man. Follow-up of iron reentry into the transferrin pool at the end of the intensive phlebotomy treatment indicates that the relative proportion of circulating CD4/CD8 cells is critically related to the regulation of iron absorption.