A prospective study of the prognostic significance of the labelling index (LI) was undertaken in 1972. The LI of the primary tumor was measured consecutively on 128 patients with mammary carcinoma at the time of initial treatment, following in vitro incubation of specimens with tritiated thymidine. The follow-up for all patients has now extended over 10 years. Relapse-free survival and total survival are significantly higher in patients with a low LI. A multivariate analysis of the prognostic factors, carried out with Cox's model, showed that LI is the most predictive indicator with respect to survival. The size of the tumor is the second most predictive indicator, while histological grading and the number of involved lymph nodes are less predictive. These results strongly suggest that proliferative activity as assessed by LI is directly related to metastatic dissemination probability, independent of other prognostic factors.