Background: Myocarditis is inflammation of the cardiac myocytes. In the first 6 months of life, myocarditis is slow-progressing and sudden death is common. It should not be forgotten that these patients can have intrauterine myocarditis. Case Presentation: A 4-month-old male patient had fast breathing and cyanosis during crying in the last month. At admission, we found tachypnea and hypoxia in his physical examination, and cardiomegaly on the chest radiogram. Subsequently, dilatation of left ventricle and left atrium, systolic dysfunction of left ventricle, and mitral valve regurgitation were found in echocardiography. The patient was diagnosed with myocarditis. We were informed that he had a twin brother, so we evaluated the second baby too. Similar but less severe findings were obtained in the echocardiographic examination of the twin brother, who had normal findings in physical examination. Infectious and metabolic scanning tests were performed on both patients. The underlying cause could not be spotted. Sizes of ventricles and atria, and left ventricle functions of the patients were found to be normal with treatment at the end of the 6th week. Conclusion: Four-month-old twins were diagnosed and recovered from myocarditis with supportive therapy in a short time. We think that the twins had intrauterine myocarditis. © 2009 by Pediatrics Center of Excellence, Children's Medical Center, Tehran University of Medical Sciences, All rights reserved.