Abstract An abnormal P wave was observed in a child affected by prolonged idiopathic ventricular tachycardia (fascicular tachycardia). After sinus rhythm restoration, the P wave was very tall and peaked (0.5 mV in lead II), suggesting a diagnosis of atrial enlargement. No cardiac abnormality, however, was detected by clinical and echocardiographic examination. The P wave abnormality lasted for about 1 month, with progressive voltage and shape normalization. These P wave changes were probably dependent on tachycardia; since the arrhythmia was long-lasting and characterized by atrioventricular dissociation, repetitive atrial contraction against closed atrioventricular valves caused stretching of atrial fibers, resulting in P wave abnormality. This observation suggests that very prolonged ventricular tachycardia may be associated with an electrocardiographic pattern of pseudoatrial enlargement.