A functional T to G germline polymorphism in the promoter region of MDM2 (SNP309) has been reported to profoundly accelerate tumor formation suggesting that it may also represent a powerful cancer predisposing allele. To investigate the role of SNP309 in cancer predisposition we undertook a case-control study of this polymorphism among 351 women diagnosed with breast cancer, 302 women diagnosed with ovarian and 258 female controls from a British population. The GG genotype was not associated with either breast cancer (OR 1.04, 95% CI 0.67-1.60) or ovarian cancer (OR 0.86, 95% CI 0.53-1.37). This study has found no evidence that the GG genotype of the MDM2 SNP309 polymorphism is associated with early onset or familial breast cancer or with ovarian cancer.