In breast cancer the study of prognostic factors has been well developed during last years and many biologic and biochemical parameters have been analyzed. Cell kinetics parameters demonstrated the capability to select patients with different risk of evolution of the neoplasia and in some cases treatment protocols are established by means of proliferation rate. The present study deals with the determination of Thymidine Labelling Index in breast tumors mostly T1-T2 with negative lymph nodes. Results demonstrated that the proliferative activity is higher in younger patients, in T2 cancers compared to T1, and in ductal infiltrating forms compared to the other more frequent histotypes. A significant correlation has been observed between TLI and nuclear grade. The preliminary analysis of the prognostic value of TLI has been two performed using an experimental cut off that is able to discriminate groups of patients with overall survival rate of 67% and 88%, depending on the different proliferative activity of the tumor.