Serum ferritin, measured by radioimmunoassay in 65 dialysis patients (mean time of dialysis 38.9 months), was significantly higher (247.9 +/- 224.5 ng/ml) than in controls (p less than 0.001), inversely correlated with dialytic age (p less than 0.05), and had no difference as far as sex or underlying nephropathy were concerned. With regard to the other hematological parameters, serum ferritin correlated inversely with total transferrin and directly with percentage of total transferrin only. In 28 patients, an intravenous iron load (0.6 g in one month) induced a further increase in serum ferritin (p less than 0.001), rather disomogenous and not clearly correlated with initial levels. The change in hemoglobin concentration after iron administration was in close relationship with initial serum ferritin levels (r = -0,72, p less than 0.001). A significant increase in hemoglobin could be detected in all the patients with serum ferritin less than 65 ng/ml and in most patients with serum ferritin less than 160 ng/ml, while hemoglobin change was negligible when serum ferritin levels in dialysis exceeded this value. Thus, although serum ferritin levels in dialysis patients may reflect not only iron overload but also an abnormal metabolism, serum ferritin measurement remains a reliable guide to iron requirement. We suggest maintaining serum ferritin levels between 65 and 160 ng/ml, to avoid iron stores depletion and on the contrary, iron overload.