In the course of regular dialyzation treatment very frequently iron deficiency develops. The objective of the work was to evaluate the participation of sideropenia in the development of the hypoproliferative component of anaemia and to evaluate the effectiveness of oral iron administration in a group of 24 dialyzed patients whose serum ferritin concentration was lower than 100 micrograms/l. Administration of 105 mg elemental iron per day (1 tablet of Ferronat retard, Spofa) for a period of 6 months led in 17 patients (71%) to a statistically significant increase of erythrocytes, serum ferritin, the iron plasma level and transferrin saturation. In these patients the mean haemoglobin value increased from 65 +/- 6 milligrams to 105 +/- 17 milligrams (increase by 40 milligrams), the mean number of red cells increased from 2.6 +/- 0.4 x 10(12)/l to 3.5 +/- 0.7 x 10(12)/l (increase by 0.9 x 10(12)/l) and the mean haematocrit increased from the initial value of 0.21 +/- 0.02 to 0.31 +/- 0.05 (increase by 0.10). In seven patients (29%) after oral substitution of iron deficiency no significant rise of any of the investigated indicators was observed. Four subjects of this group responded by a rise of red blood cells to intravenous iron administration and in the remaining three patients anaemia was favourably influenced by administration of recombinant erythropoietin (Eprex, Cilag). The results of the investigation provide conclusive evidence that iron deficiency plays a very important part in the development of anaemia in hemodialyzed patients. Substitution treatment with iron preparations extends the opportunities to treat anaemia during regular dialyzation treatment and is at the same time also very important from the economical aspect as it makes more expedient selection of patients suited for recombinant erythropoietin treatment possible.