Nineteen mural-based stromal tumors of the rectum and anal canal were reviewed, with the objective of delineating pathologic features discriminative of malignancy in these uncommon neoplasms. Ten locally excised tumors failed to recur during long-term follow-up and were considered benign. All occurred in the submucosa and ranged in size from 1.0 to 7.0 cm (mean, 2.1 cm). Three were sparsely cellular; seven had the appearance of gastric-type cellular leiomyomas. All lacked nuclear atypia and displayed mitotic activity not exceeding 1 mitosis/50 high-power microscopic fields. In contrast, of nine tumors exhibiting malignant behavior, eight (89%) were located in the muscularis propria. Their mean size was 4.5 cm (range, 1.6 to 11 cm). Necrosis was present in six tumors (67%). Seven sarcomas retained a cellular leiomyomatous appearance but exhibited moderate cytologic atypia. Mitotic counts ranged from 5 to 58 mitoses/50 high-power microscopic fields. Three locally excised sarcomas recurred in the rectum at 2, 2, and 7 years. In five patients tumor recurred in the pelvis. Five patients died of disease 0.67, 1.2, 3, 5, and 11 years post-diagnosis. One patient died with sarcoma at 31 years. Three patients are without evidence of recurrent neoplasm 5, 5, and 7 years postresection. Our data indicate that not all anorectal, mural-based stromal neoplasms are a priori malignant. While location within the muscularis propria, size, nuclear atypia, and tumoral necrosis correlate with malignancy, mitotic activity is the cardinal indicator of sarcomatous behavior in stromal neoplasms of the rectum and anal canal.