Many immune responses are controlled by genes of the major histocompatibility complex (MHC). In man these include the loci encoding the HLA-A, -B, -C, -DR, -DQ and -DP antigens, and many diseases have been linked with these. But attempts to identify HLA genes in man that might explain why an immune response against malignant tumours should be ineffective have so far been disappointing, apart from the association reported between the HLA-DR1 antigen and a susceptibility to a rare carcinoma of the thyroid gland. Here we describe another strong connection between a common malignant tumour and an HLA antigen, namely between HLA-DQw3 and squamous cell carcinoma of the cervix: from the 1988 United States tumour registry, 1 in every 63 newborn girls will develop this invasive cancer. We found that 88% of 66 patients had the leukocyte antigen HLA-DQw3 when it would normally be expected in only 50% of individuals. In animals the immune system and the MHC act in defence against virally induced tumours, but until now there has been no evidence that they do so in humans: as squamous cell carcinoma is probably virally induced, our discovery of its association with an HLA antigen will be important to the understanding of the immunogenetic basis of a susceptibility to this tumour.