The average concentration of cadmium in the air of the 28 cities for which data are available shows a marked correlation with death rates from hypertension and arteriosclerotic heart disease (coefficient of correlation [r] =0.76). This association cannot be explained by a tendency of large, industrialized cities to have both high heart disease rates and heavily polluted air, since there is no correlation with indexes of air pollution in general. Zinc, a closely related element, is the only other pollutant which correlates significantly (r=0.56). Previous studies have shown that patients with hypertension have increased cadmium levels in their renal tissue, and that small amounts can produce hypertensive disease in rats. It is difficult to explain all the current knowledge about cadmium, except by the hypothesis that it is a significant cause of hypertension and perhaps of a portion of the rest of the cardiovascular disease spectrum.