Tumour infiltrating lymphocytes (TILs) implicated in immunologic cytotoxicity were evaluated by immunohistochemistry and digitally counted in serial sections from 90 breast cancers in order to assess their number, the relationships between them and to tumour histology. CD3+, CD4+, CD8+, CD20+, CD25+ and CD56+ lymphocytes were found in 58 (64.4%), 52 (57.7%), 50 (55.5%), 22 (24.4%), 11 (12.2%) and 21 (23.3%) tumours, respectively. There was no difference in the number of TILs between pure infiltrating ductal (NOS) and non-ductal carcinomas, and no relationship between TILs and histological grades was found. CD3+ TILs directly correlated to age, while lymph node negative patients had tumours infiltrated by fewer CD4+ TILs with respect to lymph node positive patients. In 25/90 patients, randomly chosen, the status of peripheral blood lymphocytes was evaluated but no differences with respect to the status found in healthy blood donors was obtained; nonetheless while in some patients CD8+ TILs outnumbered CD4+ TILs in situ, the CD4/CD8 ratio was normal in their peripheral blood. The results show a considerable diversity of TILs among breast tumours, their lack of relationship with the status of the peripheral blood cells, and their potential important relationship with age (CD3+) and lymph node status (CD4+).