Abnormalities of the cell cycle are present in all neoplasms. Cyclin E, which regulates the G1/S phase transition of the cell cycle, plays an important role in many different cancers. To further investigate the role of cyclin E in invasive breast ductal carcinomas in South African women, representative sections from 157 mastectomy and axillary clearance specimens were stained with the cyclin E antibody. The results were compared with known clinicopathologic prognostic factors, namely lymph node metastases, size, estrogen receptor status, and histologic grade. Positive (nuclear) cyclin E immunostaining strongly correlates with negative estrogen receptor status and high grade. These correlations may account for the observation that although cyclin E staining is associated with poor prognosis in univariate analysis, no prognostic significance remains after multivariate analysis.