The duration and shape of the fetal QRS complex were studied in 88 pregnancies complicated by rhesus isoimmunization. A clear correlation between ventricular depolarization time and haemoglobin levels at birth was observed. A single QRS value greater than 4 SD above the normal mean value, or a tendency to rapid increase was very suggestive of a bad prognosis, while a QRS duration below +4 SD or declining in sequential determinations reflects a temporary compensation of fetal conditions. From the QRS complex analysis a clear indication appeared of the constant and early involvement of the heart in terms of myocardial hypertrophy and/or cardiac enlargement. The sensitivity of QRS to anaemia and the feasibility of continuous non-invasive monitoring might allow, with the determination of bilirubin concentration in amniotic fluid, improvement in the management of rhesus haemolytic disease.