Fisher, Edwin R., Palekar, Alka, and Paulson, John D.: Comparative histopathologic, histochemical, electron microscopic and tissue culture studies of bronchial carcinoids and oat cell carcinomas of lung. Am J Clin Pathol 69: 165–172, 1978. Although there is compelling evidence to indicate that bronchial carcinoids and oat cell carcinomas of lung have similar histogeneses, their prognoses are divergent, making their pathologic distinction imperative. Although this is not generally difficult, instances do occur, particularly with biopsy material, in which their absolute identification may be tenuous. The results of a systematic, statistical examination of a number, of histopathologic features of 13 bronchial carcinoids and 47 oat cell carcinomas in cases in which follow-up information for 2 to 6 years was available revealed that a ribbon-like architectural arrangement of the monomorphous tumor cells of high (most favorable) nuclear grade represents a presumptive feature of bronchial carcinoids. On the other hand, mitoses, low nuclear grade, necrosis, hematoxylinophilic vascular deposits, pericellular argyrophilic pattern, and perineural space invasion were encountered only in oat cell carcinomas. Argyrophilic granules were more frequent in cells comprising bronchial carcinoids. An argentaffin reaction and other tinctorial methods for enterochromaffin, as well as glycoprotein, were found to be positive only in bronchial carcinoids, and in the latter they were focal. Neurosecretory-type granules observed by electron microscopy were obviously more frequent in cells of bronchial carcinoids, whereas the nuclei and the cytoplasm of cells of oat cell carcinomas appeared more complex. Chromosomal analysis of cells comprising a few samples of each tumor type in which this feature was evaluated revealed aneuploid karyotypes and marker chromosomes in both. The results do not minimize the common histogenesis of bronchial carcinoids and oat cell carcinomas. However, they do provide criteria for their differentiation, as well as evidence to support the view that the bronchial carcinoid is indeed a more benign form of oat cell carcinoma of the lung.