ABSTRACT Aims Recent Japanese clinical practice guidelines for gastrointestinal stromal tumor (GIST) indicates that surgical resection is optionally considered for small ( Methods From January 2005 to April 2012, 99 patients with gastric SMTs underwent surgery. Twenty-one of them, which had tumor growth during observation period, were enrolled in the retrospective analysis. Data included patient background data, clinicopathological features, surgical outcomes and prognosis. Data were presented as median (range). Results All patients (12 males, 9 females), with age of 64 (48-74), had their lesions detected by routine health check-up. No clinical symptom was presented. Tumor was located in upper (n = 9), middle (8), and lower (4) body of the stomach, respectively. The tumor was 2.0 (0.8-4.0) cm in size at their initial detection, and enlarged up to 3.2 (2.0-7.0) cm at the end of “watchful waiting” period of 63 (8-181) months. As surgical procedure, laparoscopic partial gastrectomy was performed in 18, open partial gastrectomy in 2, and open proximal gastrectomy for 1 patient, respectively. The operating time was 115 (49-247) min, with blood loss of 20 (0-230) ml. No major complications were noted in the series. Postoperative histologic examination revealed GIST (19) and schwanomma (2). Although 14 out of 19 GISTs were categorized into “none” (1), “very low” (12), and “low” (1) risk according to Miettinen's classification, 5 cases of “moderate” risk were identified in the series. No recurrences/metastases were noted in 21.7 (0.9-75.1) months of postoperative follow-up. Conclusion Our study revealed the existence of moderate risk GISTs in asymptomatic, small gastric SMTs with size enlargement during preoperative observation. Laparoscopic surgery was safely applied to majority of those cases (85.7%). Prompt surgical intervention should therefore be considered, when gastric SMTs under watchful waiting show size enlargement. Disclosure All authors have declared no conflicts of interest.