OBJECTIVE: Prolongation of the QTc interval is a risk factor for ventricular arrhythmias and sudden death. There is a correlation between body mass index (BMI) and QTc, with longer intervals observed in obese subjects. Dieting and weight loss on selected diets are also associated with prolongation of the QTc interval, although the mechanisms and risk factors of this syndrome are not clearly established. The aim of the current investigation was to determine if short-term weight loss over seven days while living on a metabolic ward influences cardiac repolarization as indicated by QT interval duration. DESIGN AND SUBJECTS: The elctrocardiogram QT, RR, and QTc (QT/RR0.5) intervals were assessed in 30 healthy obese (BMI>27 kg/m2) men and women ingesting a balanced 1120 kcal/d formula diet (23% kcal as fat and 52 g protein/d). RESULTS: Weight loss (3.9±1.7 kg) over seven days had no measurable effect on the RR interval and there was a significant (P=0.02) shortening of the QT interval (mean±s.d., 0.395±0.028–0.386±0.027 s) and borderline significant (P=0.11) shortening of the QTc (0.411±0.028–0.404±0.025 s) interval. The QTc interval normalized in three subjects with prolonged baseline QTc (>0.44 s). A statistically significant (P=0.007) reduction in systolic blood pressure early in the course of weight loss (day 2) accompanied the electrocardiographic changes. CONCLUSION: These results fail to support the development of cardiac repolarization abnormalities in obese patients who ingest a nutritionally balanced low calorie diet over seven days. A significant reduction in QT interval duration and amelioration of abnormally prolonged baseline QTc intervals suggest improved cardiac repolarization with early weight loss.