BACKGROUND: Gastrointestinal stromal tumours (GIST) are the most common mesenchymal tumours of the gastrointestinal tract. They are defined immunohistologically as KIT positive tumours. The only effective treatment for malignant GIST was surgery until 2000. Imatinib mesylate (STI571, Glivec) has shown substantial anticancer activity in patients with metastatic or unresectable GIST. PATIENTS AND METHODS: 57 patients who were diagnosed with unresectable or metastatic malignant GIST were entered into this study. The patients were given 400 mg Glivec orally once daily. The dose could be increased to 600 mg orally once daily and then to 400 mg twice daily if tumour progression was noticed. Daily treatment was interrupted or dose was decreased only in the case of limiting toxicities. We evaluated the tumour response and the safety of the drug. RESULTS: 85% of GIST patients showed a partial response or stable disease after 8 weeks of treatment with imatinib. The main side effects were nausea, vomiting, anorexia, skin rash, periorbital oedema and diarrhea. CONCLUSION: This study confirms that imatinib is an active agent against malignant GIST with manageable toxicities.