A case of a 72-year-old woman with a cystic gastrointestinal stromal tumour (GIST) infiltrating the stomach and forming a fistula between the pathologic cavity of the tumour and stomach lumen is presented. The growth, although histopathologically malignant, produced no clinically evident metastases and developed sufficiently slowly to attain its large size and destroy the posterior part of the stomach. The final diagnosis of GIST of poor muscle differentiation was made possible due to specific immunohistochemic assays. The condition, despite the attempt of surgical treatment, was fatal. It is suggested that GISTs should be suspected in every case of a pouch-forming gastrointestinal growth as such preliminary clinical diagnosis may facilitate the use of the most proper and effective microscopic examination methods. The pathological and clinical features, diagnostic difficulties and prognosis of these rare gastrointestinal neoplasms are discussed.

Keywords: case report, gastrointestinal stromal tumors