Tumor suppressor genes are generally viewed as being recessive at the cellular level, so that mutation or loss of both tumor suppressor alleles is a prerequisite for tumor formation. The tumor suppressor gene, p53, is mutated in∼ 50% of human sporadic cancers and in an inherited cancer predisposition (Li–Fraumeni syndrome). We have analyzed the status of the wild‐type p53 allele in tumors taken from p53‐deficient heterozygous (p53+/−) mice. These mice inherit a single null p53 allele and develop tumors much earlier than those mice with two functional copies of wild‐type p53. We present evidence that a high proportion of the tumors from the p53+/− mice retain an intact, functional, wild‐type p53 allele. Unlike p53+/− tumors which lose their wild‐type allele, the tumors which retain an intact p53 allele express p53 protein that induces apoptosis following γ‐irradiation, activates p21 WAF1/CIP1 and Mdm2 expression, represses PCNA expression (a negatively regulated target of wild‐type p53), shows high levels of binding to oligonucleotides containing a wild‐type p53 response element and prevents chromosomal instability as measured by comparative genomic hybridization. These results indicate that loss of both p53 alleles is not a prerequisite for tumor formation and that mere reduction in p53 levels may be sufficient to promote tumorigenesis.