In patients with chronic renal failure, renal transplantation improves anemia and the production of erythropoietin. In patients undergoing hemodialysis the administration of recombinant human erythropoietin improves anemia with a decrease in bodily iron stores. Therefore, one would expect a similar decrease after kidney transplantation. We followed the ferric parameters to determine the incidence of iron deficiency anemia in 24 consecutive renal transplant patients for an interval long enough to achieve steady state values of hemoglobin (5.1 +/- 0.8 months). Hematological parameters and serum levels of iron, ferritin and erythropoietin were measured. The patients were divided into 2 groups according to the decrease in serum ferritin: group 1--16 with a decrease in respect to basal values (114 +/- 56 ng./ml.) and group 2--those without modifications (720 +/- 320 ng./ml.). Except for the similar values, group 1 showed greater improvement in anemia (red blood cells 4.3 x 10(6) +/- 1.1 x 10(6) versus 3.7 x 10(6) +/- 1.5 x 10(6)/ml., p < 0.01) and hematocrit index (38.5 +/- 5.2 versus 33.0 +/- 5.1%, p < 0.05). Four patients had microcythemia (mean corpuscular volume 76.6 +/- 1.4 fluid) with lower hemoglobin values than the other patients in group 1 (10.77 +/- 0.42 versus 12.79 +/- 0.42 gm./dl., p < 0.05). Among the 16 patients in group 1, 7 of 8 whose basal serum ferritin was less than 150 ng./ml. achieved ferritin levels of less than 30 ng./ml. In conclusion, our data support that renal transplantation produces a rapid decrease in iron stores and in some cases induces iron deficiency anemia. This fact should be evaluated and treated properly.