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.