A series of 42 cases of colloid carcinoma of the breast (28 pure and 14 also containing infiltrating carcinoma of non-colloid type) is reported. Pure colloid carcinomas occurred in older women, were associated with longer durations of symptoms, and tended to present as larger masses than did either the mixed colloid tumors or a comparison group of infiltrating duct carcinomas. The pure colloid tumors also differed from the other two groups in having low tumor grades, predominantly pushing rather than infiltrating tumor margins, less frequent axillary lymph nodal metastases, and better 5-year survival. The presence of noninfiltrating non-colloid carcinoma in otherwise pure colloid tumors did not change these clinical and pathologic findings to resemble those of the mixed colloid group. The authors feel that although pure colloid carcinomas form a distinct category with an excellent prognosis, there is insufficient evidence that their treamtent should be more conservative than that for other infiltrative mammary carcinomas.