Imatinib (STI571 or CGP57148B) is an innovative treatment for tumours with a constitutively activated form of c-ABL, c-KIT, or PDGFR. Such tumours include Philadelphia-chromosome-positive (Ph-positive) leukaemias, gastrointestinal stromal tumours, and PDGFR-positive leukaemias. Diseases such as primary hypereosinophilia and dermatofibrosarcoma protuberans also seem to respond to imatinib. Clinical trials assessing the therapeutic effects of imatinib have shown that the drug is highly effective with few associated side-effects, achieving durable cytogenetic responses in many patients with chronic-phase BCR-ABL-positive leukaemias. However, the emergence of resistance, particularly in patients with acute leukaemias, has prompted intense research, and many are concerned about the future prospects for imatinib. The resistance has been found in patients with acute-phase disease, but may also occur in patients with chronic-phase disease. Two cellular mechanisms for resistance to imatinib have been identified: amplification of BCR-ABL gene and mutations in the catalytic domain of the protein. In addition, suboptimum inhibition of BCR-ABL in vivo could contribute to the selection of resistant cells. We have summarised all currently available data on resistance to imatinib, both published and unpublished, including the mechanisms of resistance identified so far, and their clinical relevance to the different forms of Ph-positive leukaemias is discussed. Furthermore, we discuss strategies to overcome or prevent the development of resistance.