Mucocelelike tumor of the breast associated with atypical ductal hyperplasia or mucinous carcinoma. A clinicopathologic study of seven cases. We studied seven patients with mucocelelike tumors of the breast, known to be benign lesions that may be confused with mucinous carcinomas of the breast. All patients had a palpable mass. Microscopically, the most striking feature was the cystic character of the lesion. The epithelial lining of the cysts was usually flat or cuboidal to low columnar, and mucin pools frequently appeared near the ruptured cysts. Three mucocelelike tumors contained a microscopic focus of mucinous carcinoma. The other tumors had areas of atypical ductal hyperplasia containing abundant intraluminal mucinous materials. The mucin was composed predominantly of neutral and nonsulfated acid mucins whose character was identical to that of those in mucinous carcinoma. Because all mucocelelike tumors in our series were associated with either atypical ductal hyperplasia or carcinoma and because some mucocelelike tumors may indeed be early mucinous carcinomas of the breast, we recommend examination of the entire specimen and careful clinical follow-up. 