Pathological studies in seven hearts from patients with dilated cardiomyopathy have shown that the number of neurons is significantly reduced in these compared with five hearts from normal subjects. The number of ganglion cells was counted in a strip of right atrial wall between the venae cavae and sectioned serially. The mechanism responsible for the neuronal depopulation in this type of cardiomyopathy could not be determined. Previous viral infection may be causally related. Three hearts of patients suffering from chronic Chagas's heart disease were also studied. Depopulation of neurons was most severe in the hearts with Chagas's disease and less severe in those with dilated cardiomyopathy, though neurons were still significantly reduced in number in the latter compared with normal controls. Despite the lack of a specific, definite cause for the depopulation of neurons, physiological evidence of parasympathetic impairment in patients with dilated cardiomyopathy is in agreement with the pathological findings. It is suggested that on the basis of our findings neuronal depopulation in some patients with dilated cardiomyopathy may be of aetiological significance.