Objective: In order to elucidate the effects of radiation on centrosome hypeamplification, we examined the centrosome duplication cycle in KK47 bladder cancer cells following irradiation.Methods: KK47 cells were irradiated with various doses of radiation and were examined for centrosome hyperamplification by immunostaining for -tubulin.Results: Nearly all control cells contained one or two centrosomes, and mitotic cells displayed typical bipolar spindles. The centrosome replication cycle is well regulated in KK47. Twenty-four hours after 5-Gy irradiation, ~80% of irradiated cells were arrested in G2 phase, and at 48 h after irradiation, 56.9% of cells contained more than two centrosomes.The present study had two major findings: 1) continual duplication of centrosomes occurred in the cell cycle-arrested cells upon irradiation, leading to centrosome amplification; 2) cytokinesis failure was due to aberrant mitotic spindle formation caused by the presence of amplified centrosomes. Abnormal mitosis with amplified centrosomes was detected in the accumulating G2/M population after irradiation, showing that this amplification of centrosomes was not caused by failure to undergo cytokinesis, but rather that abnormal mitosis resulting from amplification of centrosomes leads to cytokinesis block.Conclusion: These results suggest that centrosome hyperamplification is a critical event leading to chromosomal instability following exposure to radiation.