Twenty-six cases of cystosarcoma phyllodes diagnosed at M. D. Anderson Hospital were reviewed. The following criteria were evaluated for possible correlation with local recurrence, uncontrolled local recurrence, metastasis, and tumor death: tumor size, stromal overgrowth, tumor necrosis, mitotic rate, stromal cellularity, nuclear size and pleomorphism, the presence of specialized stroma, and initial therapy. Of the 26 tumors, seven caused death. Five patients developed metastatic spread, and all of them died of tumor. Five patients had local recurrence, which was uncontrolled in three (two patients died with uncontrolled recurrence alone, and one with uncontrolled recurrence and metastasis). Stromal overgrowth was present in eight cases. Six of the seven patients who died of tumor had stromal overgrowth, including all five with metastasis. Correlation of stromal overgrowth with metastatic spread and tumor death was significant at P levels of 0.0014 and 0.02, respectively. It is concluded that stromal overgrowth is a significant histologic indicator of malignant behavior in cystosarcoma phyllodes.