The authors describe the characteristics of atypical cystic lobules (ACLs), which represent a step in the formation of low-grade ductal carcinoma in-situ. The authors define ACLs as a proliferation of luminal cells showing low-grade cytological atypia without architectural atypia. ACLs were compared with conventional hyperplasia, low-grade ductal carcinoma in-situ, and lobular neoplasia. 1) In about 40% of the cases, atypical cystic lobules merged with fully established micropapillary/cribriform ductal carcinoma in-situ. 2) Immunohistochemical staining for hormone receptors, keratin nineteen, and cyclin D1 revealed that atypical cystic lobules demonstrate a consistent immunophenotype, which differs from that of normal lobules and benign lesions and matches the one of low-grade ductal carcinoma in-situ. 3) ACLs are sometimes calcified. Osteopontin-positive histiocytes infiltrated all Kossa-positive (type II microcalcification) cribriform and comedo-type carcinomas and ACLs. The similarities in cytological and immunohistochemical features, the close proximity of the two types of proliferation, and the similarities with respect to calcification suggest that atypical cystic lobules represent an early stage in the formation of certain types of low-grade ductal carcinoma in-situ.