The immunoreactivity for Ki-67 and PCNA was investigated in 487 patients with primary breast carcinomas using MIB-1 (Immunotech, France) and PC-10 (DAKO, Denmark) as primary antibodies. Formalin-fixed and paraffin-embedded tissue sections were used. The immunoreactivity for Ki-67 and PCNA was found to be independent of the length of fixation if the sections were pretreated in a microwave oven in citrate buffer and distilled water, respectively. The immunostaining was evaluated semiquantitatively. High Ki-67 score (more than 1% positive tumour cells) and PCNA over-expression (more than 25% positive tumour cells) were correlated with clinicopathological parameters such as large tumour size, high histological grade (poor differentiation), and absence of steroid hormone receptors, which are parameters of an aggressive phenotype of the tumour. In univariate analysis of survival data, both Ki-67 and PCNA were parameters of a poor overall survival in both lymph node-positive and -negative patients. In multivariate anlaysis using a Cox model stratified by nodal status, Ki-67 and PCNA failed to be of prognostic significance whereas classical histopathological parameters such as tumour size and histological grade turned out to be of independent prognostic significance in both lymph node-positive and -negative patients, while progesterone receptors were of independent prognostic significance only in lymph node-positive patients.