Hyperplastic carotid bodies from an 80-year-old woman with sustained systemic hypertension were examined ultrastructurally and compared with those from six normal subjects. Hyperplasia involved both elongated sustentacular and Schwann's cells, both of which were more numerous than normal. They formed concentric whorls around a diminished core of chief cells. The Schwann's cells adopted a peripheral position in each cell cluster, whereas the sustentacular cells were located closer to the central core. The characteristics of both types of cell were similar so that they could not be distinguished from each other in every instance. They were associated with numerous nerve axons with which they usually displayed a simple mesaxonal relationship. The myelin sheaths formed by Schwann's cells were well developed but those encountered in sustentacular cells were usually rudimentary. A few fibroblasts were also present, but generally the process of hyperplasia was not one of fibrosis; neither did it involve pericytes.