The HLA‐G molecule is a non‐classical HLA class I antigen selectively expressed by trophoblastic cells that invade the maternal decidua during human pregnancy. HLA‐G is believed to contribute to tolerance of the semi‐allogeneic fetus by inhibiting maternal immune responses. Similarly, HLA‐G expression in tumour cells may favour their escape from host immune surveillance. This study investigated HLA‐G expression in human mammary tumours. Immunohistochemical analysis of cryo‐preserved and paraffin‐embedded breast tissue biopsies, using two HLA‐G‐specific antibodies, revealed that unlike non‐cancerous breast tissue in the vicinity of the tumour, 14 out of 36 breast cancer lesions selectively expressed HLA‐G. HLA‐G expression was significantly more frequent in lesions that were highly infiltrated by host immune cells, thus correlating HLA‐G activation with inflammation. Further histological and double‐staining immunofluorescence analysis attributed HLA‐G expression mainly to tumour epithelial cells and to subsets of infiltrating CD68⁺ and CD8⁺ cells. RT‐PCR analysis suggested that HLA‐G was activated at the transcriptional level in breast tumours. The presence of ILT2 (Ig‐like transcript 2) killing inhibitory receptors known to interact with HLA‐G was also demonstrated in host immune cells that infiltrate breast cancer lesions. These results indicate that HLA‐G is up‐regulated at high frequencies in human breast cancer, where it may impair efficient anti‐tumour immunity. Copyright © 2001 John Wiley & Sons, Ltd.