The entity described in the literature as a “distinctive cardiovascular lesion” resembling histiocytoid hemangioma and more recently referred to as “mesothelial/ monocytic incidental cardiac excrescence” may not be a true proliferative lesion. Rather, it may represent an artefact produced by suctioning of the pericardial cavity during cardiac surgery. This hypothesis was explored by comparing two index cases of cardiac histiocytoid hem-angioma-like lesions (HLLL) to (a) the contents of extracorporeal bypass pump (ECBP) filters in 22 random cardiac surgical cases, and (b) material adherent to mediastinal and pericardial drains in 15 random post-cardiac surgery cases. In 18 of the 22 ECBP filter cases (82%) and two of the 15 postsurgery cases (13%), tissue fragments indistinguishable from the HHLL index cases were identified. These filter and drain fragments had light microscopic, immunohistochemical, and ultrastructural features identical to those of the index cases, as well as to the HHLLs described in the literature. In neither index case, nor in the study patients, were any proliferative lesions identified at surgery. Three study cases subsequently came to autopsy, and no proliferative lesions were found. This and other evidence strongly suggest that these lesions not only are artefactual, but also are a common occurrence. They are likely produced during cardiac surgery by the cardiotomy suction, with compaction of friable mesothelial strips, other tissue debris, and fibrin into tumor-like fragments that may be transported around the operative site on the suction tip.