Genomic instability is often caused by mutations in genes that are involved in DNA repair and/or cell cycle checkpoints, and it plays an important role in tumorigenesis. Poly(ADP-ribose) polymerase (PARP) is a DNA strand break-sensing molecule that is involved in the response to DNA damage and the maintenance of telomere function and genomic stability. We report here that, compared to single-mutant cells, PARP and p53 double-mutant cells exhibit many severe chromosome aberrations, including a high degree of aneuploidy, fragmentations, and end-to-end fusions, which may be attributable to telomere dysfunction. While PARP(-/-) cells showed telomere shortening and p53(-/-) cells showed normal telomere length, inactivation of PARP in p53(-/-) cells surprisingly resulted in very long and heterogeneous telomeres, suggesting a functional interplay between PARP and p53 at the telomeres. Strikingly, PARP deficiency widens the tumor spectrum in mice deficient in p53, resulting in a high frequency of carcinomas in the mammary gland, lung, prostate, and skin, as well as brain tumors, reminiscent of Li-Fraumeni syndrome in humans. The enhanced tumorigenesis is likely to be caused by PARP deficiency, which facilitates the loss of function of tumor suppressor genes as demonstrated by a high rate of loss of heterozygosity at the p53 locus in these tumors. These results indicate that PARP and p53 interact to maintain genome integrity and identify PARP as a cofactor for suppressing tumorigenesis.