A patient with a myelodysplastic syndrome (MDS) and reticulocytosis of > 50% in the absence of a correspondingly increased erythrocyte turnover is reported. Evaluation of the kinetics of erythrocyte turnover revealed a decreased erythrocyte life span of 44 days. From these data a prolongation of the reticulocyte maturation time to > 20 days can be concluded. The patient's erythrocytes lacked the increase of mean corpuscular volume and a significant increase of erythrocyte enzymes that would be expected in marked reticulocytosis. This finding suggests that the reticulocytes represented not newly formed red blood cells, but "pseudoreticulocytes", i.e., mature erythrocytes that retained their substantia reticulofilamentosa.