Based on the observation of 5 patients with simultaneous breast cancer and malignant lymphoma, we discuss the phenomenon of synchronicity of two different tumors in the same individual. Synchronicity of malignancy is rare and could point to a genetic predisposition (family history) or common environmental influence in generating cancer. In elderly patients, the appearance of multiple tumors may coincide by chance. In the case of synchronous or metachronous tumors it is essential to examine the new tumor manifestation by biopsy. In view of the therapeutic and prognostic implications, the second tumor should not be confused with a progression of the known primary malignancy.