A 75-year-old man was admitted to our hospital with massive hematemesis. Upper gastrointestinal endoscopy revealed a submucosal gastric tumor with ulcer lesion, about 30 mm in diameter, in the anterior wall of the cardia. Pathological findings of biopsy specimens showed a gastrointestinal stromal tumor (GIST) of the stomach. Abdominal computed tomography (CT) showed an extramural low density mass, measuring 27 mm, in the cardia. From these findings, we thought we should be performing a wedge resection of the stomach for this tumor. After one month from the first examination, preoperative upper gastrointestinal endoscopy revealed a growth of the tumor. Abdominal CT also showed an obvious growth of the tumor, measuring 50 mm. Thus, laparoscopy-assisted proximal gastrectomy was performed, because the tumor was close to the cardia. Postoperative diagnosis of the tumor was pathologically confirmed to be a GIST with a high malignant potential. The case report of GIST that shows rapid growth for a short term is rare. We herein describe a case of GIST with rapid growth during one month before the operation.