OBJECTIVE - The C allele at the rs11212617 polymorphism in the ataxia-telangiectasia-mutated (ATM) gene has been associated with greater clinical response to metformin in people with type 2 diabetes. We tested whether this variant modified the effect of metformin in the Diabetes Prevention Program (DPP), in which metformin reduced diabetes incidence by 31% in volunteers with impaired glucose tolerance. RESEARCH DESIGN AND METHODS - We genotyped rs11212617 in 2,994 DPP participants and analyzed its effects on diabetes incidence and related traits. RESULTS - Contrary to expectations, C carriers enjoyed no preventive advantage on metformin; their hazard ratio, comparedwith A carriers,was 1.17 ([95% CI 0.96-1.42], P = 0.13) under metformin. There were no significant differences by genotype in metformin's effects on insulin sensitivity, fasting glucose, glycated hemoglobin, or disposition index. CONCLUSIONS - The reported association of rs11212617 with metformin response was not confirmed for diabetes prevention or for effects on relevant physiologic parameters in the DPP.