Carotid bodies (CBs) are peripheral chemoreceptors that can undergo compensatory hypertrophy and hyperplasia in some patients with chronic hypoxemia. At the Children's Hospital morphometric and biochemical analyses were done on CBs obtained at autopsy from 89 infants in the first year of life to determine if there were statistically significant differences in sudden infant death syndrome (SIDS, n = 38) compared with an age-matched control population (n = 51). Mean combined weights of CBs from SIDS victims were greater than controls in the 1- to 4-month age interval (mean = 3.4 mg, p less than 0.075) and the 4- to 8-month interval (mean = 5.0 mg, p less than 0.098) but the differences were not statistically significant (t-test, p less than 0.05). Computerized planimetry of total surface area and area occupied by "functional" parenchyma revealed no statistically significant differences. Both groups showed an equally intense degree of cytoplasmic argyrophilia of chief cells, and electron microscopy of CBs from 5 SIDS victims was indistinguishable from controls, including overall content of dense-core neurosecretory granules. There were also no significant differences in catecholamine content using liquid chromatography with electrochemical detection (SIDS n = 29, controls n = 21). These morphometric, ultrastructural, and biochemical data are not supportive for CBs having a significant role in the pathogenesis or etiology of SIDS.