Gastrointestinal stromal tumors (GISTs) are a group of neoplasms arising from mesenchymal stem cells of the gastrointestinal tract. The prognosis of metastatic or recurrent GISTs is poor, because these tumors resist chemotherapy and radiotherapy. We report a patient with recurrent GIST who underwent molecularly targeted therapy with imatinib, a novel oral tyrosine kinase inhibitor. A 50-year-old woman presented with a huge intra-abdominal mass. The patient had a history of gastrectomy for GIST and hepatectomy for its metastases. She also underwent surgery for resection of peritoneal metastases 9 months before. The abdominal mass was 26 x 17 x 12 cm in size, as determined by magnetic resonance imaging, and was diagnosed as a peritoneal relapse of GIST. Treatment with 400 mg of imatinib daily was started. After 1 week of treatment with imatinib, reduction of the abdominal tumor began to be recognized on palpation. Computed tomographic scanning on day 28 revealed that the tumor had liquefactively regressed and had reduced in size by 66%. The major side effect was leg edema, which was easily manageable with furosemide. The patient has been receiving imatinib treatment in our outpatient clinic, and the tumor regression has continued for 9 months. Imatinib shows promise as a safe and effective drug for the treatment of patients with recurrent GISTs.