Concentration of fetal hemoglobin (HbF) was measured in the blood of 172 cancer patients with solid tumors. The patients’ age groups were compared with matched age groups of normal controls. Above the age of 50, a significant difference was found between patients and normal controls. In 70% of the patients with active disease, the concentration of HbF was above the normal range. There were indications that by reducing HbF concentrations irradiation of patients interferes with the evaluation of the results. In addition to whole blood, HbF was also measured in the plasma of 35 patients; the average HbF concentration in this group was found to be twice as high as that of the control