We assessed the effect of intravenous methylprednisolone pulse therapy (IMPT) on cardiac rhythm and electrolyte metabolism in patients with nephrotic syndrome. A total of 25 patients had valid evaluations with continuous ambulatory electrocardiograms, and 20 of these had simultaneous sodium and potassium clearances. No significant difference of frequency in complex ventricular arrhythmias (Lown's grades 3-5) between the control and the therapy period was observed; however, 4 patients showed complex ventricular arrhythmias including ventricular tachycardia (2 patients) during the course of therapy. Fractional excretion of potassium and serum potassium significantly increased from baseline after IMPT. Complex ventricular arrhythmias, sometimes leading to sudden death, might ensue from IMPT. These dysrhythmias may be related to an abrupt change in potassium reflux from the cell.