We examined the relationship between initial blood fetal hemoglobin (HbF) concentration and prognosis in 100 children with leukemia. HbF concentration was usually elevated and ranged between 0 and 19.5 g/l. Multivariate analysis showed that, in patients with acute lymphoblastic leukemia (ALL), each increment of 1 g/l in initial HbF concentration resulted in a 1.13-fold rise in the risk of death or relapse (95% confidence limits 1.01-1.25, P less than 0.05). In patients with myeloid leukemias, each increment of 1 g/l in HbF was associated with a 1.20-fold (1.05-1.37, P less than 0.02) rise in the risk of death or relapse. In the patients with myeloid leukemias the mean initial HbF concentration was also higher (3.4 g/l; 0.5-19.5 g/l) than in the patients with ALL (1.1 g/l; 0-18.7 g/l; P = 0.08). Our results indicate that the degree of increase in HbF synthesis is associated with the degree of malignancy: the more aggressive the disease, the more augmented is the synthesis of HbF.