Serum cobalamin and homocysteine levels were studied in patients with chronic myelogenous leukemia (CML) and in stem cell donors treated with granulocyte-colony stimulating factor (G-CSF). Cytoreductive treatment in patients with CML resulted in a decrease of cobalamin and homocysteine levels. In stem cell donors cobalamin and homocysteine levels increased after G-CSF administration. The increase of homocysteine level was accompanied by a decrease in the serum levels of the cobalamin-binding protein transcobalamin. We hypothesize that the increased homocysteine levels in patients with CML and donors treated with G-CSF may be the result of a functional methylcobalamin deficiency due to decreased transcobalamin levels.