The aim of this study was to examine the expression of carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) in pancreatic endocrine tumors (PETs) and to correlate it with clinicopathologic parameters. Sixty-nine PETs were examined for tumor size, necrosis, local peripancreatic invasion and lymphovascular invasion, lymph node, and liver metastasis. The mitotic count, expressed per 10 high-power fields (HPF) and MIB1 index were assessed and tumors were classified according to the World Health Organization classification. A tissue microarray was constructed and stained with an extensive panel of endocrine markers and CEACAM1. Twenty-nine tumors were from males and 40 from females, age range: 23 to 80 years (mean 52.4 y), tumor size ranged from 0.8 to 11 cm (mean 3.5 cm), 8 patients had multiple endocrine neoplasia 1 syndrome, and 1 had von Hippel-Lindau disease. Twenty tumors demonstrated local invasion, 32 had lymphovascular invasion, 16 had lymph node metastasis, and 10 had liver metastasis. CEACAM1 was positive in 47 cases and negative in 22 cases (31.9%). Ninety percent of the CEACAM1-negative cases had a MIB1 index <or=2%, whereas 88.9% of CEACAM1-positive cases had a MIB1 index >2% (P=0.02). 86.4% of the CEACAM1-negative PETs had a mitotic count <or=2/10 HPF, whereas 75% of the CEACAM-positive PETs had a mitotic count >2/10 HPF. In addition, 80% of tumors >or=2 cm in diameter were CEACAM positive (P<0.05). CEACAM1-positive tumors were more frequently insulin negative (9 of 10 cases) (P=0.005) and vasoactive intestinal peptide-positive PETs were all CEACAM1 immunopositive (7 of 7 cases) (P=0.005). Benign tumors and PETs of uncertain malignant behavior were more frequently CEACAM1-negative and low-grade malignant cases were CEACAM1 positive (27 of 29 cases) (P=0.001). In addition, CEACAM1-positive tumors were statistically correlated with cytokeratin 19-positive tumors (P<0.05). PETs showing CEACAM1 positivity have a statistically significant correlation with several of the pathologic parameters of aggressive behavior and its overexpression is seen in those cases with increased invasiveness.