A 66-year-old female with chronic renal failure received five doses of procainamide and developed marked QT interval prolongation and recurrent episodes of torsades de pointes, which were temporally related to high serum n-acetylprocainamide (NAPA) levels and not to procainamide levels. Repeated hemodialysis was effective in lowering NAPA levels. Torsades de pointes is a potential hazard of NAPA accumulation during procainamide administration to patients with renal insufficiency.