While a considerable number of tumors of the heart have been reported in the literature, they are still relatively infrequent. Most of them have been necropsy findings, but more recently an increasing number have been diagnosed in vivo. To the purely academic satisfaction of arriving at a correct diagnosis even in cases bound to have a fatal issue, there is now added a more practical consideration. In recent years, certain cases of heart tumor have been treated with temporary or lasting effect, thus further inciting one to establish the diagnosis in good time. As a contribution to efforts in that direction we report the following case, in which, though the clinical picture had already suggested the presence of tumor, it was nevertheless the induction of pneumopericardium, in connection with roentgen examination, and the demonstration of tumor cells in the pericardial exudate, which clinched the diagnosis. Case Report The patient was an unmarried woman, 23 years old, who was admitted to the Maribo Hospital, Sept. 22, 1944, complaining of palpitation but with no other symptoms. Her health during childhood and adolescence had been good. Physical examination showed nothing abnormal. The possibility of Graves' disease was considered, but the metabolism was normal, as were the roentgen findings and the electrocardiogram. The Wassermann test was negative. A diagnosis of neurasthenia was made and the patient was discharged on Oct. 10. She continued to suffer from palpitation, which, however, did not prevent her from continuing to work as usual until November 1945, when she began to experience pain in the left side of the chest, accompanied by cough, expectoration, and fever. These symptoms regressed in the course of a few weeks, but shortly afterward nausea and vomiting occurred, with pressing pain in the epigastrium. Hepatic disease was suspected, and the patient was readmitted to the hospital Jan. 15, 1946. At the hospital the abdominal symptoms were from the beginning the object of attention. Liver function tests, however, showed normal conditions. Laboratory findings were as follows: hemoglobin 102 per cent: sedimentation rate 3 mm./hr. ; blood pressure 120/100; antistreptolysin titer 32; Mantoux I negative; Mantoux III positive; icterus index 8; no albumin, sugar or urobilinogen in the urine. No precise extension of the heart borders could be made out. The heart beat could not be felt. The heart sounds were distant but clear, without accentuation; the action was regular, about 80. Roentgen examination of the stomach (Jan. 28) showed nothing abnormal except that the distance of the fundus from the diaphragm was unusually great. The silhouette of the liver appeared to be enlarged. A chest examination showed both relative and absolute enlargement of the heart (length 18 cm., breadth 18 cm.; transverse diameter of thorax 26.5 cm.), with evidence of slight exudate in the left pleura but no congestion in the lungs (Fig. 1).