An 81-year-old woman developed pilsicainide intoxication associated with dehydration. The patient had been taking pilsicainide (100 mg/day) for 1 year because of paroxysmal atrial fibrillation. Her renal function was within normal limits. One week before admission, she was suffering from pneumonia, and had appetite loss, fever, and severe fatigue. Physical examination revealed dehydration. The electrocardiogram (ECG) on admission showed atrioventricular dissociation, idioventricular rhythm with marked QRS widening and QTc prolongation. The plasma concentration of pilsicainide on admission was markedly elevated at 6.2μg/ml, approximately 6 times the therapeutic range (0.25-1.0μg/ml). Continuous saline infusion was initiated for the treatment of dehydration,which progressively improved. As a result, sinus rhythm was recovered 2 h after admission, and the QRS and JT intervals gradually normalized. This is an interesting case because the proarrhythmia of pilsicainide was induced by dehydration. (Jpn Circ J 1999; 63: 219 - 222)