Five male Sicilian children with glucose-6-phosphate dehydrogenase deficiency were studied shortly after hemolytic crisis in order to evaluate the immediate effects of massive hemolysis on fetal Hb (HbF) levels and the number of circulating F cells. Hematological values seen 4 months after the children recovered from the crisis were considered representative of the patients' steady state. All patients had an increase in HbF levels (2.26 +/- 0.24%) and F cell number (29 +/- 4.79%) in the acute phase and their HbF values and F cells returned to normal range at control. Globin synthesis was balanced in the peripheral blood and bone marrow and there was a small peak of gamma chains. Globin chain electrophoresis showed that both G gamma and A gamma genes were active in all patients. These results confirm that hemolytic stress produces increased F cell release in peripheral blood. Such release is rapid enough (less than 72 h) to be consistent with the hypothesis of an induction of HbF synthesis in late erythroid precursors.