Using chromogranin (CG) immunohistochemical staining, the prognostic significance of endocrine differentiation was investigated in 212 patients with primary colorectal adenocarcinoma (including 6 patients with mucosal carcinoma). CG-immunoreactive cells were found to be an integral component of the tumor in 67 of 206 patients (32.5%, excluding mucosal carcinoma). The intracellular localization of CG in the CG-immunoreactive cells in cancer tissue was completely different from that in the normal endocrine cells of the large bowel. In addition, morphologic changes such as nuclear hyperchromasia and pleomorphism also indicated that the CG-immunoreactive cells in the cancer tissue were malignant. The tumors were divided into three groups based on the frequency of CG-immunoreactive cells: Group I (n = 139), negative; Group II (n = 38), less than 1 positive cell/mm2; and Group III (n = 29), more than 1 positive cell/mm2. No correlation was observed between CG-immunoreactivity (CG-IR) and tumor location, grade, depth of invasion, or stage, regardless of lymph node involvement. However, patients with numerous endocrine tumor cells (Group III) had a significantly worse prognosis compared with patients without endocrine cells (Group I) (multivariate Cox's model, P less than 0.01). Similar findings were observed in patients with node-negative tumor (multivariate Cox's model, P less than 0.05). These results indicated that the neuroendocrine differentiation is an independent prognostic factor and that CG-immunohistochemistry is useful for detecting a subgroup with a worse prognosis among patients with colorectal cancer.