A majority of colorectal adenocarcinomas displays diminished MHC class I expression, making them particularly vulnerable for NK cell-mediated killing. Generally, these tumors also show a substantial inflammatory infiltrate. Most inflammatory cells, however, reside in the tumor stroma, where they do not have direct contact with tumor cells in the tumor epithelium. In this study, we investigated the correlation between colorectal tumor MHC class I aberrations and infiltration of NK cells. We studied 88 tumor specimens obtained from 88 colorectal cancer patients for locus-specific HLA aberrations and correlated these data to infiltration of CD4, CD8+ and CD56+ lymphocytes. The lymphocyte markers were individually combined with laminin as a second marker to facilitate quantification in the different tumor compartments, i.e. tumor epithelium and tumor stroma. Locus-specific partial or total HLA class I loss was detected in 72% of the tumors studied. Twenty-eight percent had no HLA loss at all. Mean overall intra-epithelial infiltration of CD56+ lymphocytes was 7 cells/mm(2) compared to 76 cells/mm(2) for CD8 and 19 cells/mm(2) for CD4+ lymphocytes. Locus-specific partial or total loss of tumor cell MHC class I expression was positively correlated with the intra-epithelial infiltration of CD8+ cells (P = 0.01), but not with CD4+ or CD56+ lymphocytes. Triple immunofluorescence staining showed that these cells were CD8 and granzyme-B positive T-lymphocytes. Our data showed that colorectal tumors are sparsely infiltrated by CD56+ cells compared to CD8+ T-cells and that loss of MHC is associated with T-cell infiltration instead of NK cell infiltration. Considering the fact that MHC loss is quite common in colorectal cancer and that, due to local absence of NK cells, it is unlikely that there has been selection for NK-escape variants, improvement of the intra-epithelial infiltration/migration of NK cells may be an important basis for the development of an effective adjuvant NK-based immunotherapy of colorectal cancer.