Proliferating cell nuclear antigen, PCNA (PC10), immunolabelling was determined in 175 women with breast carcinomas and related to other established prognostic factors: flow-cytometric data, volume-corrected mitotic index, sex steroid receptor content and clinical outcome during the mean follow-up of 9 years. The maximum fraction of PCNA-positive nuclei (PCNA(max)), the average fraction of positive nuclei (PCNA(tot)) and the number of intensely stained nuclei per microscope field (PCNA(count)) were significantly related to histological grade (P<0.001), DNA ploidy (P<0.001), S-phase fraction (P<0.001), mitotic index (P<0.001) and sex steroid receptor content (P=0.002). PCNA(max) (P=0.015) predicted survival in univariate analysis; PCNA(tot) (P=0.025), PCNA(max) (P=0.007) and PCNA(count) (P=0.019) predicted the recurrence-free survival. In axillary-lymph-node-negative tumours, PCNA(tot) (P=0.092), PCNA(max) (P=0.036) and PCNA(count) (P=0.006) predicted survival and recurrence-free survival (P=0.011), (P=0.012) and (P=0.006) respectively. In multivariate analysis including clinical, histological, flow-cytometric and biochemical variables, PCNA(tot) (P=0.004) predicted the recurrence-free survival independently. In axillary-lymph-node-negative breast cancers, PCNA(tot) predicted accurately the patient survival (P=0.002) and the recurrence-free survival (P=0.002). The results indicate that PCNA immunolabelling has independent prognostic value particularly in local breast cancer.