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Objective: We analyzed the prognostic factors and evaluated the usefulness of mitotic index to predict the behavior of gastrointestinal stromal tumors (GIST)., Patients and Methods: Fifty three patients operated on for stromal tumors of the digestive tract were studied retrospectively. Mean follow-up was 6 years. The number of mitosis/10 high power fields was the definitive criterion for classification, regardless of their inmunohistologic differentiation. Twelve tumors had 0 mitoses, 34 from 1 to 9 mitoses, and 7 had > or = 10 mitoses. The survival rate was analyzed and the morphological characteristics and evolution were correlated according to mitotic index., Results: The incidence of advanced illness was related to the number of mitoses: 29% in the group with 1 to 9 mitoses, and 86% when there were > or = 10 mitoses. The recurrence intervals were 44 and 8 months respectively. No tumor with 0 mitoses evolved aggressively. The survival rate was significantly related (p < 0.001), to the mitotic index. The group with 0 mitoses had a survival rate of 100% after 10 years, those with 1 to 9 mitoses 69% and those with > or = 10 mitoses 14%. The other factors which influenced the prognosis (location, size, local invasion and resection) depended, as well, on the mitotic index., Conclusion: The classification of digestive tract stromal tumors by mitotic index is an efficient method because it distinguishes 3 entities with different biological behavior in the long term.