Ruptured cysts of the breast containing mucinous material may discharge secretions and epithelium into the surrounding tissues. This is a benign, little-known condition analogous to mucocele of the minor salivary glands. The age at diagnosis of six women with mucocele-like lesions (MLL) of the breast averaged 40 years (range, 25-61) and all but one were premenopausal. The lesion caused a mass in five cases. In one it was an incidental finding, and this patient had a separate unrelated nonmucinous intraductal carcinoma treated by mastectomy. One woman was treated by simple mastectomy. Four patients were treated by excision. All have remained well with follow-up of 6-88 months. The histological appearance of MLL of the breast simulates colloid carcinoma and should be considered in the differential diagnosis of such lesions. This is particularly important in young, premenopausal women, among whom colloid carcinoma is very uncommon. The contents of mammary MLL may be difficult to distinguish from colloid carcinoma in an aspiration biopsy.