Nine obese children (mean age 12.7 years, mean overweight 74.2%) were treated for 3 weeks with a very low calorie diet containing high quality protein. Eight patients (patients A) received a commercially available diet (240 kcal/1004 kJ/day) and 1 patient (patient B) a homemade dietary regimen (500 kcal/2100 kJ/day). Both preparations were supplemented with micronutrients; however, the daily intake of minerals was significantly less in patient B. All patients were monitored for the appearance of cardiac arrhythmias by frequent 24-hour Holter recordings. In patients A the mean loss of body weight was 9.4 +/- 2.4 kg, patient B lost 8.7 kg. The mean daily nitrogen balance was negative (patients A: 10.2 g/day, patient B: 6.8 g/day). Frequent blood chemistry evaluations were unremarkable. On the 14th day of treatment patient B developed arrhythmias (ventricular couplets, non-sustained ventricular tachycardias); in patients A no ventricular dysrhythmias were observed. Our data suggest that very low calorie diets containing protein of high biologic value can be associated with potentially dangerous arrhythmias.