We present the case of a twin pregnancy in which 1 fetus developed hydrops secondary to supraventricular tachycardia at 30 weeks' gestation. Transplacental flecainide administration successfully treated the condition without evidence of maternal or fetal side-effects. The case raises ethical and possibly legal issues that present when 1 fetus in a twin pregnancy develops a condition the management of which could cause complications to the other twin and/or the mother.