Abstract Background Congenital tumors, including mesenchymal rhabdomyoma, are highly infrequent. The combination of a congenital tumor and rhabdomyoma is rarer yet, even more so when primary origin is in the heart. Case presentation We present a case of fetal hydrops fetalis, wherein the post-mortem exam revealed a cardiac tumor more than twice the size of the heart itself; histological study confirmed primary rhabdomyoma involving the left ventricle. Conclusion It is essential to keep in mind that fetal tumors should always be differentiated from malformations; in fetuses tissue immaturity must be taken into account for categorization; despite benign labeling, they can be lethal.