To analyze prognostic and predictive effects of immunohistochemical factors within a randomized study of high-dose versus standard-dose chemotherapy in high-risk breast cancer with >10 involved lymph nodes. Histopathologic specimens in 188 of 302 patients were analyzed for Ki-67, p16, maspin, Bcl-2, Her2/neu, and p53. In a univariate analysis after adjustment for therapy, tumor size, and estrogen receptor, Her2/neu positivity (P = 0.001) was a negative and Bcl2 positivity (P = 0.003) was a positive prognostic factor for event-free survival. In a multivariate analysis, Her2/neu positivity (hazard ratio, 3.68; 95% confidence interval, 2.01-6.73; P = 0.0001) had a negative influence on event-free survival, whereas p53 positivity (hazard ratio, 0.57; 95% confidence interval, 0.34-0.95; P = 0.03) and Bcl2 positivity (hazard ratio, 0.35; 95% confidence interval, 0.19-0.64; P = 0.0006) were associated with a better event-free survival. Analyzing the predictive effect of the immunohistochemical factors, an interaction between p53 and treatment could be shown (P = 0.005). The hazard ratio for high-dose chemotherapy versus standard chemotherapy is estimated as 2.3 (95% confidence interval, 0.67-7.92) in p53-negative patients and as 0.46 (95% confidence interval, 0.2-1.07) in p53-positive patients, which indicates a superiority of high-dose chemotherapy in p53-positive patients and an inferiority in p53-negative patients. No interactive effect could be shown for the other factors. Her2/neu and Bcl-2 are prognostic but not predictive factors in patients with high-risk primary breast cancer; p53-positive patients might benefit more from high-dose chemotherapy than from standard chemotherapy, and p53-negative patients might benefit more from standard chemotherapy than from high-dose therapy.