Anaplastic large cell lymphoma is a rare malignancy in childhood. We describe the case of a 6-year-old boy with Ki-1 lymphoma of the thymus who presented with an endocardiac mass. The first histologic analysis suggested a high-grade undifferentiated sarcoma, but reevaluation and immunohistochemistry confirmed the CD30+ lymphoid derivation of the process. The patient was given chemotherapy and 24 months later he remains in complete remission. It is noted that echocardiography was repeated many times to detect heart lesion.