Most studies are in favor of a prognostic relevance of p53 accumulation determined by immunohistochemistry in breast cancer, but negative results are not lacking. On a series of 1400 patients with lymph node-negative cancers treated with local-regional therapy alone until relapse and with a median follow-up of 10 years, we validated the prognostic relevance for overall relapse and death of p53 accumulation observed in a pilot study and analyzed its predictivity on different adverse events. p53 protein accumulation was immunocytochemically detected using PAb1801. The case series had also been previously characterized for hormone receptor content [estrogen receptors (ERs) and progesterone receptors (PgRs)] and for cell proliferation [[3H]thymidine labeling index ([3H]dT LI)]. p53 expression, considered as a dichotomous variable with a cutoff value of 5% positive cells, significantly predicted the occurrence of overall relapse, distant metastasis, and death with an interaction with cell proliferation. p53 accumulation, cell proliferation, and their interaction term, along with tumor size and patient age, retained a predictive role for overall relapse, and together with tumor size and PgR, also for overall survival. When considered as continuous variables, we observed that the hazard of metastasis increased linearly with the increase of [3H]dT LI and decreased linearly with the increase of ER and PgR. Conversely, the hazard increased with the increase of p53-positive cells only for tumors with a [3H]dT LI lower than 7.5%. In multivariate analysis, the same prognostic factors for distant metastasis were identified when the biomarkers were considered as continuous or dichotomous variables.