A regular check-up in a 69-year-old man showed normal blood chemistry values, except for elevated value of liver enzymes and inflammatory reactions. A computed tomography scan (CT) of the abdomen revealed a normal appearance of the liver, pancreas and spleen, but incidentally showed a tumor in the right atrium. The tumor (a blood cyst which contained white thrombus) was successfully excised. Blood cysts of the heart are extremely rare in adults. These tumors are incidently found at autopsy on cardiac valves in approximately 50% of infants under 2 months of age. The blood cyst in this case arose from the right atrial wall, which is also quite rare.