Three patients with acute lymphoblastic leukemia (ALL) and heterozygous for the Mediterranean variant of glucose-6-phosphate dehydrogenase (G6PD) have been investigated with the 2-deoxyglucose-6-phosphate (2dG6P) method to determine the number and type of progenitor cells in which the disease arose. A monoclonal origin was established for the lymphoblasts, while the other hemopoietic cell lines were not involved in the leukemic process.