Accumulation of the tumour-suppressor protein p53 in breast cancer is associated with several prognostic factors that indicate an aggressive, rapidly proliferating tumour with an unstable genome. To assess p53 accumulation in stage I breast cancer and to evaluate the prognostic value of both nuclear and cytoplasmic p53, 205 patients with node-negative breast cancer and tumour size less than or equal to 20 mm were examined. Immunohistochemistry was performed on frozen sections with the monoclonal antibodies PAb 1801 and DO1. Cellular p53 accumulation, within either the nucleus or the cytoplasm or in both, showed the same association with different pathobiological variables as nuclear accumulation alone. Eleven per cent of the tumours showed strong nuclear accumulation and were significantly correlated to age under 50 years, negative oestrogen receptor status, DNA aneuploidy, high S-phase fraction, high pathological grade and poor prognosis. The distant recurrence rate ratio was 6.2 (P = 0.002). It is thus concluded that p53 accumulation is of prognostic value in early stage breast cancer.