Germ-line mutations in the TP53 gene are rare, but predispose women to a range of cancer types, including early-onset breast cancer. Breast cancers in women from families with the Li-Fraumeni syndrome often occur before age 30. The prevalence of deleterious TP53 mutations in unselected women with early-onset breast cancer is not precisely known. If mutations were found to be sufficiently common, it might be prudent to offer genetic testing to affected women in this age group. We screened the entire TP53 gene in the germ-line DNA from 95 women of various ethnic groups who were diagnosed with breast cancer before age 30, and who had previously been found to be negative for BRCA1 and BRCA2 mutations. No TP53 mutation was found. This study does not support a policy that TP53 testing should be offered routinely to unselected women with early-onset breast cancer in the absence of a family history of cancer.