This study aimed at obtaining a better understanding of the malignant characteristics of carcinoids (gut endocrinomas) of the digestive system. After excluding cases with no possible individual identification as for age and sex, 2001 carcinoid cases with metastases among 5647 of the digestive system computerized in the Niigata Registry were thus subject to the present analysis. These cases were divided into two groups: [A] 1719 with ordinary carcinoid histology and [B] 282 with atypical varieties including argyrophil (endocrine) cell carcinomas, small/oat cell carcinomas of endocrine type, composite carcinoid-adenocarcinoma varieties and other neuroendocrine tumors. In many aspects, a comparative analysis demonstrated statistical differences between [A] and [B]: average age (55.7 vs 60.1), the rates of metastases (34.4% vs 55.6%), the association of carcinoid syndrome (21.4% vs 2.8%), tumor size 20mm or less (42.2% vs 22.5%), depth of invasion down to the submucosa (21.6% vs 11.7%) and transmural invasion (34.3% vs 47.7%), immunohistochemistry of neuron-specific enolase (86.5% vs 66.7%), chromogranin (86.1% vs 67.6%), vasointestinal polypeptides, ACTH and alpha-fetoprotein, and the 5-year survival rate (61.4% vs 17.9%). No statistical differences between these two groups were found in the male to female ratio, serotonin activities and Grimelius argyrophilia. Although no generally acceptable definite criteria and definition for atypical carcinoid varieties have yet established, neoplasias of this category seem to have a wide range of histological, histochemical, immunohistochemical, ultrastructural and biochemical features that may mostly be placed in between ordinary carcinoids and ordinary carcinomas.