Regarding the size of endocrine granules in endocrine neoplasms including carcinoids, most authors have reported only the range of granule diameters, and few studies on detailed morphometry of granule sizes have been documented. The statistically comparative study of endocrine granule sizes among such neoplasms, therefore, presented many difficulties. In the present study, we investigated more detailed findings on the endocrine granule sizes by using statistical analyses of diameters of numerous (n > 200 from each tumor) endocrine granules measured randomly throughout the cytoplasm in 18 gastrointestinal carcinoids. The measured data were analyzed statistically to obtain the histogram, distribution curve, mean value, and standard deviation of granule diameters in each carcinoid. It was disclosed by the analyses that all rectal carcinoids possessed uniformly small endocrine granules in contrast to gastric and duodenal carcinoids, which showed wide distribution of granule sizes, having significantly greater diameters than those in the rectal carcinoid series. The correlation between endocrine granule sizes and producing hormone(s) was, however, not confirmed.