The relative contribution of heritable and nonheritable factors to disease expression in BRCA2 mutation carriers is largely unknown. This report describes a familial breast cancer syndrome in a pair of identical female twins. These twins showed an extremely high concordance in their clinical histories; both twins exhibited similar cancer-related risk factors, and developed breast cancer at the same age with the same disease stage and identical histological features. No differences were detected in hormone receptors status, p53, bcl-2, erbB-2 and LI Ki67 expression by immunohistochemistry. A BRCA2 exon 11 protein truncation test showed a lower molecular weight band than the one expected for a normal allele, in both twins. Sequence analysis of DNA showed a 6 bp insertion between nucleotides 4359-4360, which resulted in a premature stop codon at position 1378. The remarkable disease similarity observed in this identical twin pair is in accordance with an important role for heritable factors in disease expression among patients carrying BRCA germline mutations.