The aim of this study was to assess the antitumour response and time to progression (TTP) of patients treated with imatinib mesylate (Glivec, Gleevec, formerly STI-571) who had advanced and/or metastatic gastrointestinal stroma tumours (GIST) or other soft tissue sarcomas (STS). Patients with measurable lesions and adequate organ function were entered. They were treated with imatinib mesylate at the dose of 400 mg twice daily (bid). All tumours were subject to a stringent pathological review by an expert panel. Immunohistochemical expression of KIT expression was evaluated. A total of 51 patients (27 GIST, 24 other STS), median age 53 years, median World Health Organization (WHO) performance score 1, were entered. 71% of the patients had received prior chemotherapy. The most frequent side-effects were anaemia (92%), periorbital oedema (84%), skin rash (69%), fatigue (76%), nausea (57%), granulocytopenia (47%) and diarrhoea (47%). Most of these side-effects were mild to moderate and no patient was taken off study due to side-effects. Skin rash and periorbital oedema frequently seem to be self limiting, despite continued treatment. In GIST patients, the current response rates (RRs) are 4% complete remission (CR), 67% partial remission (PR), 18% stable disease (SD) and 11% progression (PD). 73% of GIST patients are free from progression at 1 year. In the other STS group, there were no objective responses. The median time to progression in this subgroup was only 58 days. Imatinib mesylate is well tolerated at a dose of 400 mg bid. This dose is active in patients with KIT-positive GIST, but patients with other STS subtypes unselected for a molecular target are unlikely to benefit.