An extranodal high-grade B-cell lymphoma, centroblastic type, with exclusive involvement of the heart, stomach and small bowel was detected at post-mortem examination following the death of an 80-year-old man. Autopsy revealed massive cardiomegaly with a total heart weight of 1800 g owing to an intramyocardial tumour involving the right ventricle, and multiple mucosal tumour plaques and nodules in the stomach and small bowel. The case highlights the difficulties of diagnosing cardiac lymphoma clinically even in the presence of a large tumour mass.