(1) Chronic myeloid leukaemia goes through three clinical phases: a chronic phase, an acceleration phase, and a terminal blast crisis. In the chronic phase, interferon alfa-2 is more effective than cytotoxic chemotherapies but it also has more adverse effects. (2) Imatinib inhibits tyrosine kinase, an enzyme encoded by the pathological gene BCR-ABL, which is created during a reverse translocation between chromosomes 9 and 22 (characteristic of chronic myeloid leukaemia). This translocation almost always creates the pathological chromosome Philadelphia in blood cell lines. (3) 1 027 patients were recruited to three non comparative trials of imatinib, each focusing on a different phase of chronic myeloid leukaemia. Efficacy was evaluated largely on the basis of blood cell count and clearance of cells harbouring the Philadelphia chromosome. (4) During the chronic phase, in patients in whom interferon alfa-2 had failed or been poorly tolerated, a major cytogenetic response, lasting at least one month, occurred in 35% of patients on imatinib, compared to 20% of patients on interferon alfa-2 + cytarabine (historical comparison). It is not known whether this translated into longer survival. (5) Preliminary results from a randomised but unblinded trial comparing imatinib with interferon + cytarabine seem to favour imatinib. Some patients developed relapses resistant to imatinib, owing to mutations in the BCR-ABL gene. (6) In patients going through the acceleration phase or blast crisis, imatinib did not improve survival compared with standard treatments. (7) The main adverse effects so far described with imatinib are gastrointestinal problems, oedema and fluid retention, and muscle and joint pain, which prompted patients to stop treatment in no more than 5% of cases. (8) Imatinib has a strong potential to interact with other drugs, including paracetamol, but few specific studies have been done. (9) In practice imatinib may be a useful option during the chronic phase, after interferon alfa-2 has failed or been stopped because of adverse effects, provided that its benefits, so far shown only in surrogate endpoints, translate into longer survival. During the acceleration phase and blast crisis imatinib may cause fewer side effects than existing treatments.