BACKGROUND: Hydroxyurea is used to decrease leukocyte count effectively in chronic myelogenous leukemia. We found that hydroxyurea induced apoptosis on the K562 cells via externalization of phosphatidylserine, activation of some caspase enzymes and DNA fragmentation. We would like to find another apoptotic pathway such as apoptosis associated proteins on the K562 cells. METHODS: K562 cells were grown in RPMI 1640 supplemented with 10% fetal bovine serum and treated with 40 mM of hydroxyurea. We examined the expression of Fas, Bcl2, Bax, PARP and p53 proteins by western blot and caspase-3 activity. We also examined the transcriptional factor NF-B through the electrophoretic mobility shift assay (EMSA). RESULTS: After treatment of hydroxyurea on the K562 cells, the increase of Fas protein as well as the cleavage of the PARP protein by activated caspase-3 were detected in a time dependant manner. Anti-apoptotic Bcl2 protein was decreased but apoptotic Bax protein was increased. The expression of p53 protein was not detected. The transcriptional factor NF-B was rapidly activated, reached a peak at 3 to 9 hours and gradually returned to the resting level. The induced transcriptional factor NF-B protein was composed of p50 subunits. CONCLUSIONS: We concluded that hydroxyurea induces apoptotic death of the K562 cells via the downregulation of anti-apoptotic Bcl2 protein as well as up-regulation of apoptotic Bax and degraded PARP protein in a p53 independent manner. The data also suggests that Fas expression and NF-B activation may be closely involved in hydroxyurea induced apoptotic death of K562 cells.