Objective: The clinical behavior of gastrointestinal stromal tumors is divergent. The aim of the present study was to define the clinicopathological features that determine the patient's outcome. Results: Tumors were located in the stomach (n=39, 60%), small intestine (n=22, 33.8%) and large intestine (n=4, 6.2%). Immunohistochemically, CD 117 positivity was found in 90.8%, whereas CD34, Smooth muscle actin, Desmin and S100 positivity was found in 73.3%, 61.7%, 11.7% and 28.3% of tumors respectively. All six ''CD 117-negative'' cases expressed DOG-1. The mean Ki-67 proliferation index was 8.69%±12.76. Liver metastasis was detected in seven cases. A significant association was detected between decreased mean survival time and increased tumor size (p<0.001), large bowel localization (p=0.047), mitosis (p<0.001), the presence of necrosis (p=0.001), metastasis (p=0.033), Ki-67 proliferation index (p=0.002) and risk category (p<0.001). CD 34 positivity was mostly seen in the stomach (p=0.001), and CD 34 positive tumors had longer overall survival (92.85.±5.77 months versus 67.21±13.68 months) (p=0.046). Higher Ki-67 proliferation index (≥6%) was also correlated with the presence of metastases (p=0.015). Conclusion:Our study indicates that in addition to well-known risk factors such as increased tumor size, high mitotic activity and metastasis; higher Ki-67 proliferation index, the presence of necrosis, and CD34 negativity also correlate with shorter survival time.