We aimed to assess frequency and penetrance of BRCA1, BRCA2,and TP53 mutations in women diagnosed with breast cancer aged 30 years or younger, and then correlate this frequency with family history. 17 of 36 familial cases had a BRCA1, BRCA2, or TP53 mutation, compared with three of 63 non-familial cases. The calculated population frequency of TP53 mutations was one in 5000, substantially greater than previous estimates. This finding underlines the importance of accurate elucidation of a family history from young women diagnosed with breast cancer. Establishment of family history could help with development of patient-specific management and tumour surveillance protocols.