Wolff-Parkinson-White (WPW) syndrome is a common pre-excitation syndrome. In this paper, we report a case of a 6 year-old boy with WPW syndrome who developed atypical dilated cardiomyopathy. He presented to the hospital with a cardiac murmur. Electrocardiogram revealed type B WPW syndrome, and ultrasound showed mitral regurgitation, thickening of the chorda tendinea, ventricular aneurysm with partial thinning of the interventricular septum and paradoxic motion, reduced ejection fraction, and cardiac enlargement. In addition, delayed enhanced magnetic resonance imaging detected cardiomyopathy of the intraventricular septum in the same area, and it is currently under observation as an atypical dilated cardiomyopathy. Reports of cardiac aneurysm with dilated cardiomyopathy in WPW syndrome are rare. Furthermore, persistent elevation of antistreptolysin O titer and thickening of the chorda tendinea in the present case can suggest possible association with rheumatic fever. It is unclear whether cardiac aneurysm developed by chance in WPW syndrome causing its progression to cardiac enlargement or whether it has been complicated by other diseases such as rheumatic fever, but this may be a new category of cardiomyopathy. Continued follow-up and accumulation of cases are necessary.