Breast cancer is the most prevalent type of cancer among women. CD59, a membrane complement regulatory protein, has been demonstrated to be overexpressed in most solid tumors, where it facilitates tumor cell escape from complement surveillance. However, the role of CD59 in breast cancer growth and clinical prognosis is not fully revealed. To investigate the role of CD59 in breast cancer growth and prognostic significance, we knocked down CD59 in a breast cancer cell line that is highly metastatic to the lungs, MDA-MB?231-HM. Cell growth was measured in vitro and in vivo using a xenograft model. In addition, clinical data on a cohort of 120 patients with or without lung metastasis was analyzed based on CD59 expression, which was detected by immunohistochemistry. Knockdown of CD59 significantly inhibited MDA-MB?231-HM cell growth both in vitro and in vivo. An analysis of clinical data on 120 patients revealed that patients with CD59 overexpression may have a worse prognosis. CD59 may therefore be a prognostic biomarker for poor outcome in breast cancer patients.