Imatinib mesylate is the first and only effective drug for the treatment of gastrointestinal stromal tumor at present. Mutated exon 11 of the KIT receptor is essential for the pathogenesis and response to imatinib mesylate of gastrointestinal stromal tumor; the efficacy rate (complete response+partial response) of imatinib mesylate is 53.8%, and the disease-control rate (complete response+partial response+stable disease) is 84%. Almost 90% of patients experienced non-hematological and hematological adverse effects, which were tolerable, in particular at a daily dose of 400 mg imatinib mesylate, which warranted response induction for half of the patients, and is the dose approved by Japanese medical insurance. Clinical trials suggest that an increased dose of imatinib mesylate would be beneficial, and that the interruption of imatinib treatment might result in disease progression even after a partial response. Tentative Japanese guidelines for the diagnosis and therapy of gastrointestinal stromal tumors are being prepared by the Gastrointestinal Stromal Tumor Committee of the Japan Society of Clinical Oncology, and are presented here for critical comments by colleagues.