Carcinoid tumors of the appendix are common incidental findings, but appendiceal tumors with histologic features of both carcinoids and adenocarcinomas are rare, and their biologic behavior is still unclear. We studied 10 such cases among 45 appendiceal tumors seen at Yale-New Haven Hospital between 1960 and 1982. The patients, ranging in age from 23 to 65 yr, were all clinically symptomatic [acute appendicitis (5); abdominal mass (5)]. Right colectomy was performed in 5 patients; the other 5 underwent appendectomy only. In 1 case, metastasis to a lymph node was detected; 2 patients had ovarian metastases, and 4 patients had cecal invasion. One of the 10 patients died of widely disseminated tumor, 2 are living with persistent disease, and 5 remained free of disease from 1 to 5 yr after initial surgery. Two cases were lost to follow-up. We conclude that the histology of these lesions is distinctive, enabling their differentiation from ordinary carcinoids. Because these lesions behave in a clinically more aggressive fashion than the usual appendiceal carcinoids, but are less virulent than adenocarcinoma, we support their classification as adenocarcinoids.