High dose melphalan is commonly used as a conditioning regimen for autologous stem cell transplantation in multiple myeloma. There are reports of adverse cardiac events with melphalan manifested by supraventricular tachycardia and atrial fibrillation. Here, we report a rare case of a 58 year old female with multiple myeloma, who developed sinus arrest after autologous stem cell transplantation using high dose melphalan as a conditioning regimen. It was severe and rare, therefore, monitoring for cardiac toxicity in patients receiving high-dose melphalan is mandatory.