The significance of calcium oxalate deposition in the myocardium was studied at autopsy in 21 patients with renal insufficiency. The oxalate crystals in the myocardium frequently were associated with focal necrosis and marked fibrosis. Of seven patients with severe oxalate deposits, six had extensive myocardial fibrosis. Four of these six had severe, intractable congestive heart failure. One had complete heart block and Stokes-Adams attacks, and at autopsy oxalate crystals were observed in the cardiac conducting system. These observations suggest that myocardial oxalosis is another potential complication of renal insufficiency and that oxalate-induced myocardial abnormalities may contribute to congestive heart failure and conduction abnormalities in patients with uremia.