Synchronous bilateral breast carcinoma (SBBC) and early onset are important characteristics of hereditary cases. The lifetime risk for breast carcinoma in Cowden syndrome (CS) is estimated to be 25-50%. We reported a 44-year-old woman presenting SBBC and characteristic mucocutaneous lesions of CS, confirmed by PTEN gene mutation analysis. Bilateral modified mastectomy and axillary dissection were performed. Histopathologic examination revealed a moderate-differentiated invasive ductal carcinoma with mixed features of luminal A immunophenotype (Estrogen and/or Progesterone Receptors >50% and/or Ki67 < 30% of positive cells). The skin lesions showed the characteristic findings of tricholemmoma. Lack of PTEN expression was observed in all specimens. Sequencing analysis confirmed the presence of PTEN splice-acceptor site mutation in intron 8 (c.1027-2A>G), a germline mutation which had not been previously reported in CS. The patient received adjuvant chemotherapy and tamoxifen for 5 years. After 5 years of follow-up, she persists recurrence-free. SBBC with early onset suggests a hereditary predisposition. Thus, analysis of PTEN expression abnormality, easily assessed by immunohistochemistry, may be of clinical value to screen those patients with CS.