Infiltration by lymphoid cells is a common feature of many human tumors, including breast carcinomas, and the degree of infiltration has been suggested to be a measure of the host immune response. Our analyses in a series of 1919 cases of primary ductal and lobular infiltrating breast carcinomas from women with a long-term follow-up revealed: (a) a 16-17% frequency of infiltrated tumors independent of the patient's age at diagnosis; and (b) a strong positive correlation between survival rates and the presence of lymphocytes at the tumor site in patients less than 40 years of age (P = 0.0002) but no association with prognosis in patients 40 years of age or older. Multivariate analysis indicated that lymphoid infiltration is independent of other conventional prognostic factors such as nodal status and tumor size in predicting survival. Thus, a possible immune response against the tumor seems to be relevant only in women with early-onset tumors. Because the immune system is functionally maximum in younger years, declining with age, this finding might reflect a difference in the efficiency of the immune system. Alternatively, the biology of these tumors might differ, leading to a difference in immuno-genicity.