Adoptive immunotherapy shows promise for the treatment of cancer; however, partial or mixed responses remain common outcomes due to the heterogeneity of tumours. We studied three murine mammary tumour lines that express an ovalbumin-tagged version of HER-2/neu and reproducibly undergo complete regression (CR), partial regression (PR), or progressive disease (PD) after adoptive transfer of ovalbumin-specific CD8⁺ (OT-I) and CD4⁺ (OT-II) T cells. The three tumour lines were implanted in immunocompetent C57Bl/6 host mice, and established tumours were treated by adoptive transfer of naive OT-I and OT-II T cells. Tumours of the CR and PR classes triggered almost indistinguishable T cell responses in terms of activation, proliferation, trafficking to the tumour site, infiltration of tumour stroma, and intratumoural T cell proliferation; however, tumours of the PR class showed reduced infiltration of tumour epithelium by donor T cells. PD responses were associated with early impairment of T cell activation and proliferation in draining lymph node, followed by negligible infiltration of tumour tissue by donor T cells. Histopathological determinants of outcome were investigated through an unsupervised analysis of 64 untreated tumours representing the three response classes. Tumours of the CR class had proportionately more stroma, which had a looser, more collagen-rich histological appearance. Thus, the amount and composition of tumour stroma distinguished successfully (CR) from unsuccessful (PR or PD) outcomes after adoptive T cell transfer, a finding that might facilitate the design of immunotherapy trials for human breast cancer.