9-month-old infant was inadvertently administered azithromycin 50 mg/kg, taken from floor stock, instead of the prescribed ceftriaxone. Shortly thereafter, she became unresponsive and pulseless. The initial heart rhythm observed when cardiopulmonary resuscitation was started was a widecomplex bradycardia, with a prolonged rate-corrected QT interval and complete heart block. The baby was resuscitated with epinephrine and atropine, but she suffered severe anoxic encephalopathy. Torsade de pointes and QT-interval prolongation have been reported after administration of macrolide antibiotics, including azithromycin, both intravenously and orally. This has occurred especially in the context of coadministered drugs that inhibit the cytochrome P450 (CYP) 3A4 isoenzyme, such as ketoconazole and astemizole. However, bradycardia with complete heart block has not, to our knowledge, been reported specifically with intravenous administration of azithromycin alone, either with therapeutic doses or overdose. Clinicians should be alerted about the potential of azithromycin to cause life-threatening bradycardia, and pharmacy systems should be implemented to ensure special care in the safe administration of this drug, especially when dispensed from a point-of-care source.