RATIONALE: Several family-based studies have identified genetic linkage for lung function and airflow obstruction to chromosome 2q. OBJECTIVES: We hypothesized that merging results of high-resolution single nucleotide polymorphism (SNP) mapping in four separate populations would lead to the identification of chronic obstructive pulmonary disease (COPD) susceptibility genes on chromosome 2q. METHODS: Within the chromosome 2q linkage region, 2843 SNPs were genotyped in 806 COPD cases and 779 controls from Norway, and 2484 SNPs were genotyped in 309 patients with severe COPD from the National Emphysema Treatment Trial and 330 community controls. Significant associations from the combined results across the two case-control studies were followed up in 1839 individuals from 603 families from the International COPD Genetics Network (ICGN) and in 949 individuals from 127 families in the Boston Early-Onset COPD Study. MEASUREMENTS AND MAIN RESULTS: Merging the results of the two case-control analyses, 14 of the 790 overlapping SNPs had a combined p<0.01. Two of these 14 SNPs were consistently associated with COPD in the ICGN families. The association with one SNP, located in the gene XRCC5, was replicated in the Boston Early-Onset COPD Study, with a combined p = 2.51 x 10(-5) across the four studies, which remains significant when adjusted for multiple testing (p=0.02). Genotype imputation confirmed the association with SNPs in XRCC5. CONCLUSIONS: By combining data from COPD genetic association studies conducted in four independent patient samples, we have identified XRCC5, an ATP-dependent DNA helicase, as a potential COPD susceptibility gene.