Breast cancer is the second leading cause of cancer death in North American women. There is considerable need for better prognostic markers to identify those subsets of patients who would benefit from more aggressive therapy because their tumors show an increased risk of poor clinical behavior. p27kip1 is an important inhibitor of the cell cycle that acts by binding and inactivating cyclin-dependent kinases (CDKs). The aim of this study was to determine the prognostic value of loss of p27 protein in invasive breast cancer. We performed an immunohistochemical study of 147 patients with T1 and T2 invasive breast cancers and compared survival in the high p27 expressing group with that of the low p27 expressing group. On univariate analysis comparing tumor size, nodal status, Ki-67, c-erbB-2, p53 and estrogen receptor, low or absent p27kip1 is a strong predictor of reduced disease-free survival. Importantly, on multivariate analysis, the combined effect of low p27 with high Ki-67 is a stronger predictor of reduced disease-free survival than either marker alone. This simple, reliable and inexpensive assay, particularly when used in combination with Ki-67, improves the ability to predict disease recurrence and could become a useful adjunct of breast cancer evaluation to better identify high risk patients.