In a long-term study of 150 unselected patients with carcinoid tumors of the appendix, we found that the neoplasms were usually less than 1.0 cm in largest dimension and discovered as an incidental finding during surgery performed for other reasons. Metastases were observed with none of the 127 appendiceal carcinoids less than 2.0 cm in largest dimension, with 3 of the 14 lesions greater than or equal to 2.0 cm but less than 3.0 cm in greatest dimension, and with 4 of the 9 lesions greater than or equal to 3.0 cm. Paradoxically, the patients with the larger tumors and metastases were younger than those with smaller and clinically benign tumors. The median age of patients with tumors greater than or equal to 2.0 cm was 31 years and that of those with metastases was 29 years, as compared with a median age of 42 years in patients with nonmetastasizing tumors less than 2.0 cm. We conclude that simple appendectomy is adequate treatment for patients with apparently localized tumors less than 2.0 cm in largest dimension. We have not observed any recurrences or metastases among 122 such patients followed for a median time of more than 26 years. Simple appendectomy is probably also appropriate treatment for lesions greater than or equal to 2.0 cm in elderly patients or in those at high operative risk. Right hemicolectomy seems justified only in young patients with tumors greater than or equal to 2.0 cm who have a low risk of operative morbidity or mortality. Vascular involvement and invasion of the mesoappendix are features that may favor a more radical approach.