Lesions of the breast containing extravasated mucin span a continuum from benign mucoceles to invasive mucinous (colloid) carcinoma. It is well known that distinguishing benign from malignant mucinous lesions is difficult in fine-needle aspiration material. Whether these lesions also are difficult to distinguish in core needle biopsy material is not known. To address this, I reviewed the results of 4,297 breast core needle biopsies. Mucinous lesions were identified in 22 cases (0.51%), and excisional biopsy material was available for 15 of these. At excision, 0 of 8 benign mucinous lesions showed carcinoma, while 7 of 7 mucinous lesions associated with carcinoma at the time of core needle biopsy showed carcinoma at excision. The vast majority of mucinous lesions of the breast can be diagnosed accurately by core needle biopsy. Whether all such lesions require excision is not known at this time.