To further characterize the clinicopathologic spectrum of colorectal neuroendocrine neoplasia, 24 carcinomas with neuroendocrine differentiation were subtyped as either small cell neuroendocrine, oat cell variant (six cases), small cell neuroendocrine, intermediate variant (16 cases), or moderately differentiated neuroendocrine carcinoma (two cases). Five oat cell variants, 14 intermediate variants, and two moderately differentiated tumors were studied with antibodies to cytokeratin, vimentin, epithelial membrane antigen (EMA), neuron-specific enolase (NSE), chromogranin (CRG), synaptophysin (SYN), neurofilament, S-100 protein, carcinoembryonic antigen (CEA), and Leu-7. All tumors were immunoreactive for cytokeratin and the majority were also positive for EMA and NSE. Positivity for specific neuroendocrine markers was uncommon, with SYN reactivity noted in one oat cell variant and four intermediate variants, and CRG positivity observed in four intermediate variants and one moderately differentiated tumor. Ultrastructural analysis of four oat cell, eight intermediate, and one moderately differentiated tumor revealed neurosecretory-type, dense-core granules in all lesions, except two oat cell variants studied from paraffin-retrieved material. Hepatic and regional lymph node metastases were noted in five of six oat cell, eight of 16 intermediate, and two of two moderately differentiated tumors. Of 17 patients with follow-up (four oat cell, 11 intermediate, and two moderately differentiated tumors), only two individuals were alive after 1 year. There were no detectable differences in survival or response to treatment between morphologic subtypes. The prognosis of colorectal neuroendocrine carcinoma appears worse than for adenocarcinoma of comparable stage. Their distinction is thus warranted, especially in regard to the intermediate variant and moderately differentiated tumors, which may be potentially misinterpreted as forms of adenocarcinoma.