[A case of familial breast cancer--identical twin sisters with a heterochronic bilateral breast cancer that developed at the same age].

We have encountered very unique cases which involve identical twin sisters who underwent surgical treatment for a unilateral breast cancer at the same age of 28. Each, after 13 years, developed cancer in the opposite breast. In addition yet another sister, the youngest, had to undergo the same surgical procedure, at the same age that her identical twin sisters had had their first operations. These findings may indicate the existence of factors which decide the age of the malignancy onset, in addition to the existence of a predisposing factor for carcinoma in the genes of breast cancer patients.