One proinflammatory cytokine possibly implicated in the pathogenesis of asthma is IL-1beta. We analyzed the polymorphism of the IL-1beta gene (single nucleotide exchange at position -511) in a cohort of 245 patients with asthma (mean age, 58 years; age range, 31-84 years) and 405 controls. There were no differences in the allele frequencies. However, in men the genotype distribution differed significantly between asthmatic and control subjects (P =.03). The number of 1.2 heterozygotes was decreased in the patient group (P =.01). In subgroup analysis, the genotype distributions between cases and controls proved significantly different only in men with lower serum IgE (<100 IU/mL; P =.009). The odds ratio of the 1.2 heterozygote men was 0.37 (95% CI, 0.19-0.71). Thus in the case of men the effect of the IL-1beta locus is clear, whereas in women there is no effect. In men this might make a significant contribution to the total asthma prevalence because of the high number of functional genotypes (ie, 50/50 IL-1beta homozygote/heterozygote ratio).