Using laser microsurgery and cell fusion we have explored how additional centrosomes and/or chromosomes influence the duration of mitosis in human cells. We found that doubling the chromosome number added approximately 10 min to a 20 min division, whereas doubling the number of centrosomes added approximately 30 min more. Extra centrosomes and/or chromosomes prolong mitosis by delaying satisfaction of the spindle assembly checkpoint. Thus mitosis can be prolonged by non-genetic means and extra chromosomes and centrosomes probably contribute to the elevated mitotic index seen in many tumours.