BACKGROUND: Gastrointestinal stromal tumours (GISTs) are rare tumours of the gastrointestinal tract characterised by cell-surface expression of the tyrosine kinase KIT (CD117). No effective systemic treatment is available. Imatinib (STI571) inhibits a similar tyrosine kinase, BCR-ABL, leading to responses in chronic myeloid leukaemia, and has also been shown to inhibit KIT. We did a phase I study to identify the dose-limiting toxic effects of imatinib in patients with advanced soft tissue sarcomas including GISTs. METHODS: 40 patients (of whom 36 had GISTs) received imatinib at doses of 400 mg once daily, 300 mg twice daily, 400 mg twice daily, or 500 mg twice daily. Toxic effects and haematological, biochemical, and radiological measurements were assessed during 8 weeks of follow-up. 18Fluorodeoxy-glucose positron-emission tomography (PET) was used for response assessment in one centre. FINDINGS: Five patients on 500 mg imatinib twice daily had dose-limiting toxic effects (severe nausea, vomiting, oedema, or rash). Inhibition of tumour growth was seen in all but four patients with GISTs, resulting in 19 confirmed partial responses and six as yet unconfirmed partial responses or more than 20% regressions. 24 of 27 clinically symptomatic patients showed improvement, and 29 of 36 were still on treatment after more than 9 months. PET scan responses predicted subsequent computed tomography responses. INTERPRETATION: Imatinib at a dose of 400 mg twice daily is well tolerated during the first 8 weeks, side-effects diminish with continuing treatment, and it has significant activity in patients with advanced GISTs. Our results provide evidence of a role for KIT in GISTs, and show the potential for the development of anticancer drugs based on specific molecular abnormalities present in cancers.