Risk factors for QTc interval prolongation and dispersion, indicators of an increased risk for cardiac arrhythmia and sudden death, have been investigated in patients with eating disorders (ED) and ongoing weight loss. Patients were characterized with regard to weight, body mass index (BMI; weight/length2), duration of weight loss, rate of weight loss and rate of weight loss immediately preceding examination. At examination, a 12-lead electrocardiographic (ECG) registration and blood samples for analysis of serum electrolytes were obtained. In total, 92 examinations in 58 female patients aged 15.5+/-1.7 (mean +/- SD) y were analysed. Control ECG recordings were obtained from 38 normal-weight teenage girls with no known heart disease. Patients with ED weighed 40.7+/-7.8 kg, corresponding to BMI 15.2+/-2.4 kg/m2 following a weight loss of 11.8+/-6.5 kg. In ED patients, the ECG showed bradycardia, a shift to the right of the QRS axis, diminished amplitudes of the QRS complex and T wave, and prolongation and increased dispersion of the QTc interval. In multiple regression analyses low weight, low BMI and rapid weight loss immediately preceding the examination were the most important independent predictors of QTc interval prolongation and dispersion. It is concluded that an ECG examination is an important part of the assessment of patients with ED and ongoing weight loss, even in the absence of electrolyte disturbances, and especially if the patient is severely underweight or weight loss is rapid.