Not all patients with metastatic breast carcinoma (MBC) in a sentinel lymph node (SLN) have metastasis in additional axillary nodes (ANs). A biological marker that can predict this occurrence may be beneficial in triaging only appropriate patients for AN dissection (AND). Our aim was to study p16 expression in SLNs and to determine whether it is a predictor of metastases to additional ANs and a marker of poor prognosis. We correlated p16 expression in SLNs and ANs of 54 patients with MBC with clinicopathologic features and the nodal proliferative index (PI). We sequenced p16 from DNA in 7 cases. We found that 35 of 54 cases (65%) had p16-positive tumor cells. Nine of 17 (53%) cases in which both SLN and AND were done had MBC in additional ANs. The SLNs of 8 of 9 cases (89%) were p16 positive (73% positive predictive value). Eight of 17 (47%) cases had no metastases in ANs even though their SLNs had metastases. The SLNs of 5 of 8 (62.5%) of these cases were p16 negative (83% negative predictive value). Ductal MBCs were p16 positive in 27 of 37 cases (73%). Carcinomas with a lobular component were p16 negative in 9 of 11 cases (82%). Nine of 12 (75%) p16-negative ductal carcinomas were estrogen receptor (ER) positive. Some 75% of T2 and T3 tumors were p16 positive, compared with 50% of T1 tumors. The highest PI (defined as > or =50%) was seen in p16-positive SLNs (5 of 6 cases). The p16 DNA sequence was normal, and no mutations were found. Our findings indicate that p16 expression in SLNs with MBC predicts (1) increased likelihood of metastasis in additional ANS, and its expression along with other markers and clinicopathologic parameters may serve as an indicator for proceeding to a formal AND; (2) poor prognosis and is associated with larger primary tumors with a high nodal PI and ER-negative status; and (3) histological subtypes. Gene mutations were not responsible for the expression of p16 in our cases.