Hydroxyurea is used in the treatment of sickle cell anemia and beta-thalassemia major to increase the content of hemoglobin F (HbF) and presumably ameliorate clinical symptoms. Under therapy with hydroxyurea an increase of the mean corpuscular volume (MCV) of the erythrocytes can be observed. To evaluate a possible estimation of the content of HbF using the increase of MCV under treatment with hydroxyurea, we measured MCV and HbF during therapy with hydroxyurea. The median MCV before therapy was 87.8 fl (range 74.3-95.7) and under hydroxyurea 104.1 fl (81.0-139.5), and the median HbF 1.8% (0.1-5.4). Although both MCV and HbF increased under treatment with hydroxyurea, a linear correlation between these two parameters was not detectable. Therefore, MCV cannot replace the measurement of HbF.