Thirty-six patients with persistent cardiomegaly of indeterminable origin were studied to observe the effects of prolonged bed rest on heart size. All patients previously had exhibited little or no response to conventional treatment. Treatment consisted in the institution of prolonged bed rest and continuation of previous therapy, including digitalis and diuretics. The over-all response was good. Eleven of 21 patients who completed the prescribed course of bed rest showed return of heart size to normal, four showed definite decrease in heart size, and six experienced no change. Four deserters and an additional 11 patients currently on prolonged bed rest experienced the same level of response during incomplete observation periods. All patients showed marked clinical improvement.