HLA-G is a non-classical MHC class I gene with a limited tissue distribution. The most pronounced expression is detected in the cytotrophoblast of first trimester placenta. It is possible to detect mRNA for HLA-G in preimplantation blastocysts where expression is correlated with a high cleavage rate of embryos. HLA-G seems to play an important role in the feto-maternal relationship. The polymorphism of the HLA-G locus is not fully clarified. One study has shown extensive nucleotide sequence variation in the exon 3 (alpha-2 domain) in healthy African Americans. A few studies in other populations have only revealed a limited polymorphism. We investigated the polymorphism of the exon 3 of HLA-G by means of Polymerase Chain Reaction (PCR)-Single Strand Conformation Polymorphism (SSCP)- and DNA sequencing analysis in a Danish population. We detected four single-base substitutions in exon 3 compared to the sequence of HLA-6.0 (G*01011); one of these has not been reported before. We also found a deletion of the first base of codon 130 or the third of codon 129 in a heterozygous individual. This study, together with previous results, suggests that the polymorphism of exon 3 of the HLA-G gene in Caucasians is limited, in contrast to that observed in Americans originating from Africa. Implications of this discrepancy and the detected deletion in relation to certain disorders of pregnancy are discussed.