Several studies have investigated the associations between X-ray repair cross-complementing group 3 (XRCC3) Thr241Met polymorphism and the susceptibility to lung cancer and bladder cancer, but results have been inconclusive. In order to derive a more precise estimation of the relationship, a meta-analysis was performed. A total of 22 case control studies, including 2976 cases and 4495 controls for lung cancer, and 3445 cases and 4599 controls for bladder cancer, met the inclusion criteria and were selected. Overall, there was no evidence showing a significant association between XRCC3 Thr241Met polymorphism and lung cancer risk. Furthermore, the results for bladder cancer showed that significant decreased risk was found for the additive model (odds ratio [OR] = 0.959, 95% confidence interval [CI], 0.924-0.996) and dominant model (OR = 0.982, 95% CI, 0.963-1.000) but not for the recessive model (OR = 0.958, 95% CI, 0.905-1.014). In summary, our meta-analysis indicates that XRCC3 Thr241Met polymorphism may be weakly associated with the risk of bladder cancer. (Cancer Sci 2010; 101: 1777-1782 L ung cancer is one of the most common cancers worldwide, and it is the leading cause of cancer-related deaths in the world.(1) The role of genetic susceptibility in lung cancer has shown that the relatives of patients with lung cancer had an increased risk of the disease.(1,2) Only a fraction of smokers and a low number of non-smokers develop lung cancer, which implies influence of host factors in individual susceptibility. This inter-individual difference in susceptibility may be attributed to genetic polymorphisms in critical genes, including those involved in DNA repair. (3,4) Bladder cancer is among the most frequent diagnosed cancer in the developed world.(5) Although development of bladder cancer is associated with exposure to tobacco and occupational exposure, (6) only a small proportion of exposed individuals will develop cancer, suggesting the involvement of genetic factors.DNA repair systems play an critical role in maintaining genomic integrity.(3) If DNA damage is unrepaired, mutations are propagated during subsequent cellular replication and ultimately result in activation of oncogenes or inactivation of tumor suppressor genes. So mutations on these genes which alter the function of these proteins may predispose an individual to cancer. Increasing molecular epidemiologic evidence has shown that polymorphisms in various DNA repair genes are associated with an increased risk of cancer. (7,8) The X-ray repair cross-complementing group 3 (XRCC3) belongs to a family of genes responsible for repairing DNA double strand breaks caused by normal metabolic processes and ⁄ or exposure to ionizing radiation. The XRCC3 is involved in homologous recombination repair (HRR) and chromosomal double-strand breaks repair processes, and it is necessary to maintain genomic integrity. It was demonstrated that cell lines defective in XRCC3 had a 25-fold decrease in homology directed repair of DNA double-strand breaks.(9) Shen et al.ide...