The relationship between the size of the primary tumour upon initial treatment and the incidence of distant metastasis during the course of the disease was investigated using data from 2648 breast cancers treated at the Institut Gustave Roussy between 1954 and 1972. This analysis suggests the existence for each tumour of a critical volume (threshold) at which the first remote metastasis is initiated. The correlation between the size of the primary tumour and the probability of metastatic dissemination was assessed as well as the influence on this correlation of two prognostic indicators: histological grade and number of involved lymph nodes. It was found that the threshold volume is strongly correlated with the number of involved lymph nodes and the histological grading.