Protein p21Cip1/Waf1 is transcriptionally activated by the tumour suppressor p53 and previous studies have shown that p21 plays a role in tumour suppression. However, the involvement of p21 in p53-mediated tumour suppression remains to be directly demonstrated in vivo. Tumour suppression mediated by p53 can be measured by comparing tumour susceptibility in animals carrying two (wild-type mice) or three (super-p53 mice) copies of the p53 gene. We have taken advantage of this genetically defined system to measure p53-mediated cell-cycle arrest, apoptosis and tumorigenesis, in a p21 wild-type and in a p21-null context. The absence of p21 significantly impaired the enhanced p53-mediated cell-cycle arrest characteristic of super-p53 cells, but did not affect the enhanced apoptosis. Importantly, in an experimental model of fibrosarcoma induction, the absence of p21 significantly decreased the tumour suppression benefit of super-p53 mice. We conclude that cell-cycle arrest through p21 plays a significant role in mediating p53-dependent cancer protection.