We investigated the effect of calorie and protein content of very-low-calorie diets for weight loss upon myocardial electrical activity. Patients were followed on very low calorie diets for up to 20 weeks. Study No. 1 had 16 obese patients (4 men and 12 women). Study No. 2 had 113 obese women. Both study groups were consecutive samples with patients who had BMI > 30. In Study No. 1 patients were given 400 kcal/d with 50 gm of protein and 45 gm of carbohydrates over 12 weeks. In Study No. 2 patients were given either Metabolic Nutrition Program (MNP) which contained 600 kcal/d with 70 gm protein and 70 gm carbohydrates, or Optifast-70 which contained 420 kcal/d with 70 gm protein and 32 gm carbohydrates, for up to 20 weeks. Electrocardiograms were obtained at baseline and during the third month in Study No. 1, and monthly in Study No. 2. The sum of the voltage in leads I+II+III and the length of the QT interval were derived from the electrocardiograms. In Study No. 1 the QRS voltage decreased from 2.77 +/- .18 mv to 2.45 +/- .17 mv (p < .02). Patients lost 18.1 kg +/- 4 kg and the QT interval remained unchanged. In Study No. 2 at 20 weeks the QRS voltage decreased on Optifast-70 (p < .001) in comparison to MNP, in which the QRS voltage remained unchanged. Patients on Optifast-70 lost 17.5 +/- .53 kg, patients on MNP lost 18.5 +/- .66 kg and the QT interval in both groups remained unchanged. The formula diets tested of less than 600 kcal/d were associated with a decrease in electrocardiographic voltage.