We report a case of synovial sarcoma of the heart in a 14-year-old boy, who presented with signs of mild generalized illness and rapidly progressed to haemodynamic collapse. On operation the tumour was attached to the tricuspid valve, away from the atrioventricular node. He was considered to have incomplete resection due to extensive adhesions to the atrioventricular junction, and was offered chemotherapy. On 18 months follow-up, he was asymptomatic.