Eleven tumors of the carotid body and I chemodectoma of the ganglion nodosum of the right vagus nerve were encountered in six members of two consecutive generations of a family. All affected members had bilateral tumors of the carotid bodies with a single exception; one member had in addition a similar neoplasm occurring in the ganglion nodosum of a vagus nerve. Four of these patients had associated chronic obstructive pulmonary disease and exhibited persistently high arterial pCO2 and low pO2 levels. The hypothesis is advanced that chemodectomas of the carotid bodies and other chemoreceptive structures begin as hyperplasias initiated by variations of these physiological stimuli in individuals unable to maintain their normal values or control their oscillations, perhaps because of genetically mediated incompetence. Evidence from the literature supporting this interpretation is discussed. Cancer 33:1635–1641, 1974.