It is unknown whether an association exists between infectious microorganisms and atherosclerosis. Eighty consecutive patients undergoing carotid endarterectomy were studied to detect for bacterial or virus infections in removed carotid atherosclerotic plaques. Twenty-one patients (25%) were found to have positive cultures for bacteria of the carotid plaques. Three patients (4%) did not have bacterial contamination of controlled cultures of the skin. Of these three patients, two grew diptheroids and one grew staphylococcus. The control cultures of the skin demonstrated that 25 patients (31%) grew diphtheroids and 29 (36%) grew staphylococcus. Five patients grew both organisms. There was no evidence of colonization within the atheromatous plaque material in histologic studies of the three patients that had positive cultures of their plaque. All viral cultures were negative. The positive carotid cultures found were most likely due to contamination from the skin. This study demonstrates the unlikelihood of bacterial or virus infections as either an etiologic or a pathogenetic factor in carotid artery atherogenesis.