Small cell carcinomas of the large intestine. The authors studied the clinical and pathologic features of 38 small cell carcinomas of the large intestine. Most were located in the right colon. Overlying adenomas were present in 45% and squamous differentiation in 21% of tumors. Endocrine differentiation was present in all tumors by at least one method; neuron-specific enolase, dense-core granules, and synaptophysin were present in most cases. Seventy-one percent of tumors metastasized to the liver; 64% of patients were dead at five months follow-up. Twenty-one poorly differentiated adenocarcinomas and undifferentiated carcinomas of the large intestine accessioned during the same period showed less endocrine (7 of 21) and squamous differentiation (1 of 15) and fewer liver metastases (4 of 15) than did small cell carcinomas. Among all 59 tumors studied, small cell histologic characteristics correlated better with liver involvement than did endocrine markers or other histologic features. Small cell carcinomas of the large intestine are aggressive tumors with a propensity for early liver involvement. Although there is a spectrum of squamous, endocrine, and glandular features in large bowel tumors of low degrees of differentiation, the identification of a small cell component appears to be most clinically relevant. 