Background: Asthma is a complex polygenic disease in which gene-environment interactions are important. The gene encoding tumour necrosis factor alpha (TNF alpha) is one of several candidate loci for asthma pathogenesis and is highly polymorphic. A number of studies have investigated the polymorphism of TNF alpha-308 gene promoter ( substitution G -> A, designated as TNF1 and TNF2) in relation to asthma susceptibility in different populations. However, the results of individual studies have been inconsistent.Methods: To address the inconsistent findings in studies of the association of the polymorphism of TNF alpha-308 gene promoter with susceptibility to asthma, a systematic review was undertaken of the published data and a meta-analysis was performed. The MEDLINE database was searched for case-control studies published in English language journals from 1966 to October 2005. Data were extracted using standardised forms and pooled odds ratios (ORs) with 95% confidence intervals (CIs) were calculated.Results: Fifteen eligible studies, comprising 2409 patients with asthma and 3266 controls, were included in the meta-analysis. Using the random effects model, the pooled result showed that the TNF2 allele is associated with overall susceptibility to asthma (OR 1.37, 95% CI 1.02 to 1.84, p = 0.04). The ORs for asthma susceptibility in TNF2 homozygote individuals were significantly increased at 2.01 (95% CI 1.26 to 3.20, p = 0.009) and 1.51 (95% CI 1.02 to 2.22, p = 0.041) compared with TNF1 homozygotes and TNF2/1 heterozygotes, respectively. In addition, the pooled OR for asthma risk in TNF2/1 heterozygotes was also significantly higher than that in TNF1/1 homozygotes (OR 1.47, 95% CI 1.01 to 2.13, p = 0.045).Conclusions: The TNF2 allele confers a significant risk for developing asthma. A large scale case-control study is needed to clarify the functional effect of the polymorphism of the TNF alpha gene in the pathogenesis of asthma.