Proliferating cell nuclear antigen (PCNA), was examined by immunohistochemistry in 509 breast carcinomas. The immunoreactivity was found to be independent of the length of fixation when the tissue sections were microwaved before incubation with the primary antibody. The PCNA immunoreactivity was assessed by two semi-quantitative methods, which were correlated but not exchangeable. The comedo type of intraductal carcinomas and invasive ductal carcinomas had a higher PCNA score than other types. Lymph node metastases had a significantly higher PCNA score than primary carcinomas. High PCNA immunoreactivity was correlated with the presence of lymph node metastases, absence of tubule formation, numerous mitoses, severe nuclear pleomorphism, high histological grade and absence of progesterone receptors (PgR). PCNA in lymph node positive tumours was correlated with tumour type, especially with ductal carcinomas, absence of tubule formation, high histological grade and absence of PgR, whereas PCNA in lymph node negative tumours was correlated with large tumour size, numerous mitoses, severe nuclear pleomorphism and high histological grade. Number of mitoses and nuclear pleomorphism were the two most important factors in predicting the PCNA score; the absence of PgR and nuclear pleomorphism were important in lymph node negative and positive tumours, respectively. In a univariate analysis high PCNA score was found to be correlated with shorter relapse-free period and poorer over-all survival.