Eight adrenocortical adenomata from patients with Cushing's syndrome were examined by light and electron microscopy. Light microscopically, the reticulin fibers were markedly prominent in areas of the compact-type cells and ran tangentially to the adenoma cells or parenchymal cells. Electron microscopically, adrenocortical adenoma was composed of parenchymal cells, interstitial cells, and endothelial cells. In areas of the clear-type cells, the parenchymal cells were arranged in long columns and covered by the sinusoidal wall. In areas of the compact-type cells, the sinusoid was well-developed and meandering. The dilated intercellular space was conspicuous and filled with filamentous substance and collagen fibrils. The parenchymal cells appeared to produce the filamentous substance and collagen fibrils, and underwent fragmentation. Interstitial cells and fragmented cytoplasm of the parenchymal cells were found in the intercellular and perisinusoidal space. It was likely that the fragmented cytoplasm flowed into the sinusoid. Experimental study using cell culture methods revealed these phenomena, especially a production of collagen fibrils.