Thiamine responsive megaloblastic anemia syndrome (TRMAS) is a rare, autosomal recessive disorder characterized by megaloblastic anemia, diabetes mellitus, and progressive sensorineural deafness. Mutations in the SLC19A2 gene that codes for thiamine transporter 1 protein cause TRMAS, and more than 30 homozygous mutations have been identified to date. Congenital heart diseases and arrhythmias have been reported in few patients. We present cardiac features of five patients with TRMAS. Five patients had macrocytic anemia, diabetes mellitus, and sensorineural deafness. Two siblings had also optic atrophy. SLC19A2 gene mutation was shown in all patients. Two patients developed supraventricular tachycardia during an episode of diabetic ketoacidosis. Five patients had absent P waves on baseline electrocardiography, and one patient had additional low QRS voltage. None of the patients had structural heart disease. Discontinuation of thiamine treatment appears to trigger supraventricular tachycardia episodes at puberty.