Imatinib inhibits the BCR-ABL tyrosine kinase created by the Philadelphia chromosome (Ph+) in chronic myeloid leukaemia (CML). Complete haematological responses were achieved in 88% of patients and major cytogenetic responses were detected in 49% of patients with chronic phase CML treated with oral imatinib 400 mg/day in a multicentre noncomparative study of 532 patients. Administration of oral imatinib 400 or 600 mg/day to 235 patients with accelerated phase CML in a multicentre noncomparative study resulted in haematological responses in 63% of patients and major cytogenetic responses in 21% of patients. 26% of the 260 patients with blast crisis CML receiving imatinib 400 or 600 mg/day in a multicentre noncomparative trial sustained a haematological response and 13.5% of patients had a major cytogenetic response. Imatinib 400 or 600 mg/day orally achieved ahaematological response in 19 of 32 patients with Ph+ acute lymphoblastic leukaemia in a pilot study. Clinical improvement was demonstrated in 89% of 36 patients with gastrointestinal stromal tumours unresponsive to standard chemotherapy during treatment with 400 or 600 mg/day oral imatinib in a noncomparative phase II trial. Adverse events were frequent in clinical trials of imatinib but most events were mild or moderate in severity. Serious adverse events reported include severe fluid retention, cytopenias and hepatotoxicity.