Infiltration of CD3⁺CD8⁺ cytotoxic T cells was analyzed by multiparameter confocal laser microscopy in a panel of 16 randomly selected stage I nonsmall cell lung carcinomas. T-cell infiltration was observed in the stroma (range 57-,093 T cells/mm²) but also in the tumor epithelium (range 21-92 T cells/mm²) and showed wide variation between individual tumors. Interestingly, a significantly higher percentage of CD3⁺CD8⁺ T cells was detected in the tumor epithelium compared to the stroma illustrating that cytotoxic T cells may preferentially migrate into tumor epithelium. Aberrant HLA class I antigen expression was observed in 69% of the nonsmall-cell lung carcinoma (NSCLC) tumors. One tumor of a squamous cell lung carcinoma patient with the highest number of tumor infiltrating CD3⁺ and CD3⁺CD8⁺ cells was studied in detail and the majority (90%) of these cells were shown to be functionally activated granzyme B-positive cytotoxic T cells. DNA oligotyping of a lung carcinoma cell line established from this tumor revealed loss of one HLA haplotype corresponding with a translocation involving chromosome 6, as observed by COBRA-FISH. HLA class I-restricted tumor specific T cells could be isolated from PBMC. One further characterized cytotoxic CD8⁺ T cell clone, that released TNF-α, IFN-γ, and granzyme B upon co-incubation with the autologous tumor cells, was shown to be restricted by the remaining HLA-A11 allele, which was also shown to be expressed in the tumor tissue. Our data indicate that, despite HLA-haplotype loss a vigorous antitumor immune response mediated by CD8⁺ T-cells can be present in NSCLC offering possibilities for specific immunotherapy.