The effect of fructose-1,6-diphosphate (FDP) on left ventricular function was assessed in seven patients with chronic ischemic heart disease and eight patients with idiopathic dilated cardiomyopathy. In a crossover study design each patient received 10 gm of FDP or saline placebo intravenously for three days. An M-mode echocardiographic assessment of left ventricular (LV) function was made before and after each treatment period. After FDP treatment, LV end-diastolic and systolic dimensions showed a 6% reduction (P less than 0.01), while peak lengthening rate of LV dimension in diastole and peak shortening rate of LV dimension in systole increased 17% and 10%, respectively (P less than 0.05). There was evidence that FDP was more effective in the patients with ischemic heart disease than in the patients with cardiomyopathy.