Abstract. Activated leukocyte cell adhesion molecule (ALCAM) is a membranous cell adhesion protein that is often expressed in breast cancer. Data on the prognostic impact of ALCAM expression is highly controversial in this cancer. To evaluate the clinical impact of ALCAM expression in a sufficiently large patient cohort, we utilized a tissue microarray (TMA) containing more than 2,100 primary breast cancers with clinical follow-up data by immunohistochemistry. TMA spots containing normal breast epithelium showed moderate to strong membranous ALCAM staining. ALCAM staining was strong in 66.2%, moderate in 10.9%, weak in 11.1% and absent in 11.8% of 1,778 (80.9%) interpretable breast cancer tissue spots. Decreased ALCAM expression was significantly associated with advanced tumor size (p=0.0017), unfavorable tumor grade (p<0.0001), negative ER and PR status (p<0.0001 each) as well as high Ki67 labeling index (p<0.0001). Cancers with ACLAM expression loss had a significantly poorer overall (p<0.0001) and disease-specific survival (p= 0.0088). This association also held true in the subset of nodal positive cancers (p<0.0001). In conclusion, these data demonstrate that ALCAM is generally expressed in normal and cancerous breast epithelium and that a marked reduction of ALCAM expression characterizes a subset of breast cancer patients with adverse tumor characteristics and unfavorable clinical outcome.