BACKGROUND The objective of this study was to clarify the prognostic and predictive value of immunoreactivity for the cyclin‐dependent kinase inhibitor p27(Kip1) in patients with early‐stage breast carcinoma and to investigate its relation with clinicopathologic features and other markers. METHODS Immunoreactivity for p27 protein was analyzed on tumor slides from 461 patients who were enrolled in the International Breast Cancer Study Group (IBCSG) Trial V (median follow‐up, 13 years), including 198 patients with lymph node negative disease and 263 patients with lymph node positive disease. Tumors with < 50% immunoreactive neoplastic cells were considered low expressors. Immunoreactivity for p27 was correlated with several clinicopathologic characteristics. Disease free survival (DFS) and overall survival were analyzed according to p27 immunoreactivity and treatment group. RESULTS In the lymph node negative population, decreased p27 immunoreactivity was associated with higher tumor grade (P = 0.001) and HER‐2/neu overexpression (P = 0.04). In the lymph node positive population, low p27 expression was associated with higher tumor grade (P = 0.01), low expression of thymidylate synthase (P = 0.001), and higher Ki‐67 expression (P = 0.007). DFS was not significantly different according to p27 status in either lymph node negative patients (10‐year DFS: low p27 expression, 53% ± 5%; high p27 expression, 55% ± 5%) or in lymph node positive patients (10 year DFS: low p27 expression, 33% ± 4%; high p27 expression, 32% ± 4%). However, in the lymph node negative population, the benefit of one course of perioperative chemotherapy with cyclophosphamide, methotrexate, and 5‐fluorouracil was confined exclusively to patients with tumors that showed reduced p27 immunoreactivity (P = 0.03; test for interaction). CONCLUSIONS This analysis indicates that p27 immunoreactivity has little if any prognostic value in patients with early‐stage breast carcinoma. However, these findings suggest that, in patients with breast carcinoma who have negative lymph node status, reduced p27 immunoreactivity is associated with HER‐2/neu overexpression and may be predictive of a benefit from the early administration of adjuvant chemotherapy. Cancer 2003;97:1591–600. © 2003 American Cancer Society. DOI 10.1002/cncr.11224 Reduced immunoreactivity of the p27 protein has been associated with a poor prognosis in patients with early‐stage breast carcinoma. This study showed that there is no difference in prognosis and treatment response according to p27 expression; although, in patients with negative lymph node status, reduced p27 expression is associated with HER‐2/neu overexpression and may predict a benefit from the early administration of adjuvant chemotherapy.