Abstract: G1/S transition defects have been a proposed requirement for tumor development. Apocrine metaplasia (APM) in the breast has been held as a sign of benignity. Yet, a number of studies have reported the presence of molecular abnormalities in some forms of APM suggesting a possible oncogenic potential for some of these lesions. We currently investigate the role of some of the cell cycle proteins, previously reported to be de-regulated in breast cancer, in an attempt to assess their significance in APM. Using immunohistochemistry, the expression of cyclin D1, cyclin A, p27, p21, p16, pRb and Ki-67 was examined in 93 cases of APM. The cases were divided into nonpapillary (NAPM) (30 cases) and papillary metaplasia (PAPM) (63 cases). PAPM was further subdivided into simple papillary (SPAPM) (29 cases), complex papillary (28 cases) and highly complex papillary (six cases). For statistical analysis, the last two groups were merged together (CPAPM). The results showed that increased histological complexity was associated with significant increase of proliferative capacity and alterations of the cell cycle control. The median Ki-67 index was 1.5% in SPAPM and 4.8% in the CPAPM. Also, alterations of the cell cycle regulators were significantly higher in the CPAPM than in the SPAPM. NAPM was generally similar to normal breast epithelium. Apocrine cells were negative for p16 while pRb was expressed in all cases. These findings suggest that in complex forms of APM, there is a considerable degree of cellular unrest. This may contribute to increased susceptibility to carcinogenesis.