The tumour suppressor p53 transcriptionally regulates a range of target genes that control cell growth and survival. Mutations of p53 have been implicated in the development of approximately 50% of human cancers, including those instigated by exposure to mutagens. Although numerically rare, cancers can arise as a consequence of inherited mutations, such as in the Li-Fraumeni syndrome, which is caused by mutation of one p53 allele. Gene-targeted mice deficient for p53 have been generated to study this familial cancer syndrome. On a C57BL/6 background, p53-deficient mice develop primarily thymic lymphoma and more rarely sarcoma. Evasion of apoptosis is considered to be essential for neoplastic transformation. As proteins of the Bcl-2 family are the critical regulators of apoptosis, we investigated the role of the pro-survival members Bcl-2, Bcl-x(L) and Bcl-w in cancer development in p53(+/-) and p53(-/-) mice by testing whether ABT-737, a pharmacological inhibitor of these proteins, could prevent or delay tumourigenesis. Our studies showed that ABT-737 prophylaxis only caused a minor delay and reduction in γ-radiation-induced thymic lymphoma development in p53(-/-) mice, but this was accompanied by a concomitant increase in sarcoma. These data show that, collectively, Bcl-2, Bcl-x(L) and Bcl-w have only minor roles in thymic lymphoma development elicited by defects in p53, and this may indicate that Mcl-1 and/or A1 may feature more prominently in this process.