Gastrointestinal stromal tumor is a mesenchymal tumor of the digestive tract. Although there used to be no effective therapy for the tumor, there have been many recent reports on the efficacy of imatinib. We report on a 53-year-old female patient with a primary tumor of the jejunum who underwent 3 operations. As the tumor could not be removed at the 3rd operation, she was given imatinib orally. Results showed significant reduction ratios of the tumor area (83.0%) and volume (92.2%) at 18 months after starting imatinib administration. Also, the mean reduction ratios of the tumor area and volume per month (%/M) after starting imatinib treatment showed remarkable results, especially during the initial 3 weeks: 53.9%/M and 49.5%/M, respectively. Whether imatinib is the first choice of treatment for GIST or not, and what is the appropriate dose and period should be resolved.