BackgroundThe aim was to present a 35-year-old female patient with diagnosis of monophasic primary pericardial synovial sarcoma (PSS) with cytopathological findings.Case PresentationThe case with back pain, palpitation and weakness, was diagnosed with pericardial effusion and suspicious mass adjacent to right heart in ultrasonography. Computerized tomography showed mass 12 × 11 × 6.5 cm in size, located in right mid-anterior pericardial area, with heterogeneous internal structure, heterogeneously contrasting right heart and prominent pressure on superior vena cava. Cytopathology of pericardial effusion showed monotonous cells with oval-spindle vesicular nuclei, less amphophilic cytoplasm, evenly distributed chromatin and inconspicuous nucleoli. The pericardial mass was resected incompletely, spindle cell mesenchymal tumor with hypercellular fascicular structure and with infiltrative margins, containing a small amount of loose myxoid stroma, occasionally necrotic areas was observed histopathologically. Immunohistochemical positive reaction was for vimentin, Bcl-2, TLE-1. Accordingly, the case was diagnosed with monophasic PSS.ConclusionsThis case of monophasic primary PSS was an extremely rare malignancy diagnosed with the cytopathological findings.