Fetal hemoglobin (HbF) screening was performed on 296 cancer patients. In almost 80% of the patients with active disease (n = 197) and 35–60% of those with inactive disease (n = 99), the concentration of HbF in the blood was above the normal range. Among patients with metastatic breast carcinoma (n = 27), 89% had a high HbF concentration. The HbF level was significantly higher (p < 0.001) in patients with active disease than in those with inactive disease. There is evidence of an ectopic production of HbF into the plasma of patients, but it will be necessary to develop a method for the prevention of hemolysis in order to establish this claim.