Interactions between genetic and environmental determinants are likely to be important in the pathogenesis of chronic obstructive pulmonary disease. We hypothesized that interferon gamma (IFNG) single nucleotide polymorphisms (SNPs) and their interaction with smoking are associated with the rate of decline or level of lung function in smokers. We studied four SNPs in IFNG in 585 non-Hispanic whites (NHW) who had the fastest (n =280) or the slowest (n=305) decline of FEV(1)% predicted selected from among continuous smokers followed for 5 years in the NHLBI Lung Health Study. We also studied 1061 NHW with the lowest (n=530) or the highest (n=531) baseline lung function at the beginning of the LHS. Two SNPs were associated with baseline levels of lung function and the p values were 0.008 for +2197T/C in a dominant model and 0.002 for +5171A/G in a recessive model. However, after adjusting for confounding factors, only +5171A/G was still significant (p=0.001 for the recessive model). In addition, there was a significant genotype and smoking interaction with p=0.006 for the +5171A/G (GG vs.GA + AA) for the baseline lung function. When comparing individuals with GG versus individuals with AG + AA for low lung function, the adjusted odds ratios decreased significantly as pack-years increased. No association was found in the rate of decline study. There was an association between IFNG genotype and baseline of lung function and this association was modified by cigarette smoking.