It is known that coexistence of extreme thrombocytosis and thrombosis is observed frequently in chronic myeloproliferative disorders. In this paper, we report a patient who was prediagnosed with chronic myeloproliferative disorder; however, later on the diagnosis was confirmed as metastatic gastric cancer. The patient was a 75-year old female who was admitted with pain, swelling and erythema on her right arm. White blood cell count was 20 ×109/l, hemoglobin 5.2 g/dl and platelet count 1 088 ×109/l. Doppler ultrasonography revealed acute thrombotic process in right brachial vein. Since brachial vein thrombosis, thrombocytosis and anemia were seen together, the presumed diagnosis for this patient was chronic myeloproliferative disorder, and therapy with hydroxyurea, allopurinol and enoxaparin was started. One day after the heparin treatment, hematemesis and melena occurred. Eusophagogastroduodenal endoscopy showed an ulcerated and hemorrhagic polipoid lesion extending towards the cavity in the cardia region. Hydroxyurea treatment was stopped since the myeloproliferative disorder was excluded. Abdominal ultrasonography and tomography proved multiple metastatic lesions in the liver. Gastric and liver biopsies revealed “well differentiated adenocarcinoma” and 5-fluorouracil chemotherapy plus folinic acid was planned. As a result, in cases with thrombocytosis and thrombosis, metastatic cancers should be kept in mind besides chronic myeloproliferative disorders.