Background/Aims: The origin and chnical behavior of gastrointestinal stromal tumors (GIST) have been a source of recent speculation and controversy. This study was performed to determine the classification and prognostic factors of GIST. Methods: The clinicopathologic and immunohistochemical features for the presence of smooth muscle actin (SMA) and S-100 protein (S-100) in the 29 cases with GIST were reviewed. Results: The ratio of male to female among 29 cases was 20:9 and the median age was 59 (range, 38-73). The operations were performed in the 27 cases (2 cases, wedge resection; 6 cases, partial gastrectomy; 12 cases, total gastrectomy; and 1 case, biopsy). In immunohistochemical study using the antibodies of SMA and S-100, 9 cases showed the immunoreactivity to antibody of SMA. Two cases showed immunoreactivity to antibodies of both SMA and S-100, but 13 cases did not show the immunoreactivity to either antibodies. The positively stained group to the antibody of SMA had the tendency of consisting of spindle shaped cells and low mitosis numbers. The overall 5 year survival rate was 50%. Tumor size, presence of lymph node metastasis, distant metastasis, and degree of nuclear atpypism affected on the survival rate. Conclusions: Tumor size (> 10 cm vs 10 cm), presence of lymph node metastasis, distant metastasis and nuclear atypism could be prognostic factors. Local excision with adequate surgical margins seems to show similar outcome compared with conventional gastric resection. The results of immunohistochemical study showed the possibility of diverse cellular origin of GIST in the stomach. (Kor J Gastroenterol 1998;32:320 - 331)