Recently, cell size, cell density, and growth pattern were found to be reliable histologic parameters in separating benign from malignant duodenal stromal tumors. However, there are few data on the histologic features and important prognostic parameters of stromal tumors from other parts of the small bowel. Thus, we studied the clinical and pathologic features of 39 stromal tumors of the jejunum and ileum to determine which parameters would be most useful in distinguishing a benign from a malignant tumor. In all cases, the following histologic parameters were recorded: (a) predominant growth pattern (organoid, fascicular, solid, or mixed), (b) cellularity (low or high), (c) predominant cell type (spindled, epithelioid, or mixed), (d) nuclear pleomorphism (minimal, moderate, or severe), (e) the presence or absence of tumor cell necrosis, (f) the presence or absence of mucosal infiltration, (g) the presence or absence of skeinoid fibers, and (h) the number of mitotic figures per 50 high-power microscopic fields (HPF). Clinical follow-up was obtained in all cases, and the patients were considered to have suffered an adverse outcome if they developed metastatic disease or died as a complication of their tumor. In the absence of these features, patients were not considered to have suffered an adverse outcome. Twenty-five patients suffered an adverse outcome. Twenty-one patients died of disease from 1 month to 9 years (median: 2 years). One patient died at 4 days because of postoperative complications. Three patients were alive with metastatic disease at 6 months, 6 years, and 7 years. Twenty-four of these 25 patients developed metastatic disease, most commonly to the liver. Fourteen patients did not suffer an adverse outcome. Eleven patients were alive without disease from 2 to 11 years (median: 3 years), and three patients died of unrelated causes at 1, 1, and 3 years. Although there was some overlap in features between clinically benign and malignant tumors, features that were significantly associated with an adverse outcome included tumor size > 5 cm, mitotic counts > 5 mitotic figures per 50 HPF, high cellularity, the absence of a predominant organoid growth pattern, the absence of skeinoid fibers, the presence of severe nuclear pleomorphism, and the presence of mucosal infiltration and tumor cell necrosis (p < 0.05 using the chi-square and Fisher's exact tests). Features that were significantly associated with decreased survival included tumor size > 5 cm, mitotic counts > 5 mitotic figures per 50 HPF, high cellularity, the absence of skeinoid fibers, and the presence of tumor cell necrosis (p < 0.05 using the Mantel-Haenszel log-rank test). Given the fact that there is some overlap in these features between clinically benign and malignant tumors, a multiparametric analysis using the above features is the most effective way of predicting clinical behavior.