SUMMARY Granulocytic sarcomas are rare extramedullary tumour-like proliferates of myelogenous precursor cells. Rarely, these tumours may de novo precede acute leukaemia or represent the initial manifestation of a blast crisis in the course of a chronic myeloproliferative disease. A case of a 53 year old male who was admitted to hospital due to intestinal bleeding combined with weight loss is reported. General examination, laboratory studies and imaging findings were nonspecific. Colonoscopy revealed a rectal polypoid mass. Microscopic pathology and immunohistochemical findings were consistent to that of a collision tumour composed of rectal adenocarcinoma arisen on a pre-existing adenomatous polyp and granulocytic sarcoma. Two months later he was readmitted to hospital and the diagnosis of acute myetoid leukaemia was established. The patient finally died due to infection. Coexistence of rectal adenocarcinoma and granulocytic sarcoma should be recognized promptly so that proper therapy is initiated, thus improving prognosis. Keywords: granulocytic sarcoma, rectal adenocarcinoma, acute myeloid leukaemia, immunohistochemistry.