We have determined whether genetic variability in the gene encoding interleukin 1 receptor antagonist (IL1Ra) contributes to individual differences in susceptibility to the development of preeclampsia. The study involved 133 preeclamptic and 112 healthy control pregnant women who were genotyped for the variable copy number of 86-bp tandem repeats in intron 2 of the IL1Ra gene. Chi-square analysis was used to assess genotype and allele frequency differences between the preeclamptic and control groups. The frequencies were also compared with those in the general population. The frequency of the A2 allele was 38% among women with preeclampsia and 33% in the healthy control women (P=0.18), which was close to the rate in the general population (29%). The distribution of the A2 alleles and pooled homozygous and heterozygous genotypes having this allele was significantly different between the general population and the study group (P=0.006), at an odds ratio of 1.51, whereas the case-control setting was not sufficiently powerful to show a significant association. Using population frequencies as a reference, an association between IL1Ra polymorphism and preeclampsia was apparent, although the clinical importance of this finding is not clear.