An adrenocortical adenoma associated with adrenogenital syndrome in a two-year-old boy was investigated light and electron microscopically. Urinary 17-ketosteroid excretion was considerably elevated and unresponsive to dexamethasone administration. The level returned to normal after surgical removal of the tumour. Adenomatous cells display striking cellular and nuclear pleomorphism. Megalocytes with huge nuclei and nucleoli frequently occur. Deep cytoplasmic indentations cause nuclear pseudoinclusions and bizarre shape of the nuclei. True nuclear inclusions are also seen, as well as nuclear fragmentation. Cytoplasmic organelles show striking morphological alterations. Mitochondria with lamellar and tubular cristae are transformed into round or ovoid organelles of vesicular type. Their internal compartment is reduced, matrix material increases relatively, and mitochondrial inclusion bodies develop. Mitochondrial inclusions are identified as corresponding to fuchsinophil (siderophil or argyrophil) granules seen in the light microscope. Their staining properties indicate their glycoprotein nature. Vesicular profiles of smooth endoplasmic reticulum predominate and stacks of rough endoplasmic reticulum are transformed into tubules and vesicles. In Golgi regions, only vesicular elements are enriched. Lipid droplets are scarce. It was not possible to demonstrate histochemically catalase activity in microbodies. Dense bodies only occur in small, undifferentiated tumour cells. Multivesicular bodies, autophagosomes and residual bodies are rare. Lipofuscin is absent. Tumour cells are thought to derive from a population of undifferentiated cells ("germinative tumour cells"). Their morphological features and organelle equipment during a hypothetical course of differentiation and following dedifferentiation is described and discussed with respect to exceeding androgen synthesis.