Daily urinary colony-stimulating factor (CSF) output was tested in 16 patients with chronic myeloid leukemia, 12 of which were in stable phase and 4 in blast crisis of the disease. The CSF of stable-phase patients varied within normal range, while all patients in blast crisis had values lower than normal. Also the number of CFUC ranged within normal values in stable-phase patients, while no colony growth was detectable in blast-crisis patients. Sequential studies (28 or 11 months) of 2 patients revealed an inverse relation between daily urinary CSF and presence of peripheral immature or blast cells in the patient in stable or blast phase, respectively.