This is the 12th reported case of benign mesothelioma of the atrioventricular (AV) node, studied both clinically and pathologically. In this case, serial sections were done throughout the entire conduction system. The patient clinically showed AV block with a nodal pacemaker. Pathologically the approaches to the AV node and most of the node were replaced by tumor. The AV bundle was not involved by tumor. This is a small, slow growing tumor probably derived from epicardial rests. It has never been diagnosed clinically, but could be surgically removed before the AV block is complete.