To investigate whether the peculiar cardiac histiocytoid hemangioma-like lesions (HHLL) or mesotheliallmonocytic incidental cardiac excrescences (MICE) described in earlier studies were artifactual. The contents of extracorporeal bypass (ECBP) filters and material adherent to mediastinal and pericardial drains were examined in 37 cases of cardiac surgery and compared with two index cases.