Deceptive bizarre stromal cells in polyps and ulcers of the gastrointestinal tract. The clinical and pathologic features of 33 pseudomalignant lesions of the gastrointestinal tract with bizarre stromal cells are reported. Deceptive histologic changes were identified in ulcers of seven patients and in inflammatory polyps of 26. A misdiagnosis of malignant neoplasm was made in six of the 33 patients (three polyps and three ulcers). A history of gastrointestinal bleeding and/or inflammatory bowel disease was common. The bizarre stromal cells were usually distributed beneath the ulcerated mucosa or within granulation tissue. They stained strongly for vimentin in 20 of 23 cases. Some of the bizarre cells also stained for muscle specific actin (seven of 23 cases). The cells appear to be reactive fibroblasts or myofibroblasts. Follow-up information obtained on 24 of the 33 patients (including four of the six cases initially diagnosed as malignant) revealed 22 patients to be alive without evidence of a malignant neoplasm (average follow-up, 13 months). Two patients died of other causes. Correct recognition of these bizarre stromal cells in gastrointestinal ulcers and inflammatory polyps will prevent a potentially serious diagnostic pitfall. 