Aberrations in centrosome numbers have long been implicated in aneuploidy and tumorigenesis, but their origins are unknown. Here we have examined how overexpression of Aurora‐A kinase causes centrosome amplification in cultured cells. We show that excess Aurora‐A does not deregulate centrosome duplication but gives rise to extra centrosomes through defects in cell division and consequent tetraploidization. Over expression of other mitotic kinases (Polo‐like kinase 1 and Aurora‐B) also causes multinucleation and concomitant increases in centrosome numbers. Absence of a p53 checkpoint exacerbates this phenotype, providing a plausible explanation for the centrosome amplification typical of p53−/− cells. We propose that errors during cell division, combined with the inability to detect the resulting hyperploidy, constitute a major cause for numerical centrosome aberrations in tumors.