Allergic asthma is a chronic inflammatory disease in which interleukin-18 (IL-18) plays an important role. However, there are controversial reports on IL-18 promoter polymorphism as an independent marker of asthma susceptibility. The aim of the present study was to examine the IL-18 promoter polymorphism in patients with allergic asthma. Two hundred and thirty-one patients with allergic asthma from a Polish population diagnosed according to the National Heart, Lung, and Blood Institute (NHLBI)/WHO guidelines were examined. An allele-specific polymerase chain reaction was used to analyse polymorphisms at positions −137 and −607 in the promoter region of the IL-18 gene. Neither in the −607 C>A nor in the −137 G>C promoter polymorphism were there any differences observed between the total group of asthmatic patients and the controls in the frequencies of genotypes, alleles, diplotypes or haplotypes. In patients with severe asthma, the −607 CC and −137 GG genotypes were observed significantly more frequently (P = 0.03 for both), whereas in patients with mild and moderate asthma, the −137 CC genotype was more prevalent than in the former group. The strongest difference between mild to moderate and severe asthma was observed in −137 allele frequencies (P = 0.006). The results of the present study suggest that the −137 G allele and the C-G/C-G diplotype seem to be involved in the pathogenesis of the severe form of asthma.