It is well known that early-onset breast cancer may be due to an inherited predisposition. When evaluating women diagnosed with breast cancer under age 30, two important syndromes are typically considered: Hereditary Breast and Ovarian Cancer Syndrome and Li-Fraumeni syndrome. Many women are offered genetic testing for mutations in the BRCA1 and BRCA2 genes; however, few are offered genetic testing for mutations in the TP53 gene. There is a concern that overly restrictive testing of TP53 may fail to recognize families with Li-Fraumeni syndrome. We reviewed the genetic test results and family histories of all women with early-onset breast cancer who had genetic testing of the TP53 gene at the Toronto Hospital for Sick Children. Of the 28 women tested, six (33.3 %) had a mutation in the TP53 gene; a mutation was found in 7.7 % of women who did not meet current criteria for Li-Fraumeni syndrome. By reviewing similar data published between 2000 and 2011, we estimate that 5-8 % of women diagnosed with early-onset breast cancer, and who have a negative family history, may have a mutation in the TP53 gene. Given the potential benefits versus harms of this testing, we discuss the option of simultaneous testing of all three genes (BRCA1, BRCA2, and TP53) for women diagnosed with breast cancer before age 30.