A rapid assay for serum ferritin was performed in old and new patients undergoing regular dialysis treatment. The mean values were higher in the patients, especially the older patients, than in the controls. The normal differences in serum ferritin with age and sex were absent in the patients. We attribute these changes to iron loss and multiple transfusions received. Tissue iron was well reflected by serum ferritin concentration even during iron treatment, unless the intravenous route was used. Although an acceptable rise of serum ferritin was obtained, results of the iron administration have been poor.