BACKGROUND: Human breast cancers progressively grow despite the presence of extensive lymphocytic infiltration and specific antitumor immune recognition, thereby calling into question the competency of breast tumor-infiltrating lymphocytes (TIL). The function of breast TILs in vivo and their possible role in the suppression of an antitumor immune response are largely unknown. METHODS: The cytokines produced in situ by lymphocytes in 89 breast carcinomas and 14 benign breast lesions were assessed using immunohistochemistry. RESULTS: The majority of tumor and benign breast samples contained T-cell infiltrates, which were disclosed using an anti-CD3 antibody stain. The percentage of tumor samples in which > or =3% of the lymphocytes were producing cytokines was as follows: interleukin (IL)-2 45%, IL-4 36%, tumor necrosis factor-alpha (TNF-alpha) 28%, transforming growth factor-beta 1 (TGF-beta 1) 20%, IL-10 11%, interferon-gamma (IFN-gamma) 4%, and granulocyte-macrophage colony-stimulating factor (GM-CSF) 3%. Production of IL-2, IL-4, and TGF-beta 1 by TILs in breast cancers exceeded that detected in benign breast lesions (p < 0.005). Significantly more tumor samples contained lymphocytes producing IL-2, IL-4, TGF-beta 1, and TNF-alpha than IFN-gamma and GM-CSF (p < 0.002 for each comparison). One or more of the potentially immunoinhibitory cytokines-IL-4, IL-10, or TGF-beta 1-were produced by lymphocytes in 44% of the specimens. No significant associations were seen between lymphocyte production of a particular cytokine and disease-free survival (median follow-up 43 months). CONCLUSIONS: Immunohistochemical techniques can be used to detect cytokine secretion by TILs in preserved tissue. The relative lack of secretion of IFN-gamma and GM-CSF, rather than a deficiency of IL-2, may explain why the antitumor immune response to breast cancer is impaired.