Infantile hypertrophic pyloric stenosis (IHPS) is a severe condition characterized by hypertrophy of the pyloric sphincter muscle. We conducted a genome-wide association study (GWAS) on 1,001 surgery-confirmed cases and 2,401 controls from Denmark. The six most strongly associated loci were tested in a replication set of 796 cases and 876 controls. Three SNPs reached genome-wide significance. One of these SNPs, rs11712066 (odds ratio (OR) = 1.61; P = 1.5 × 10(-17)) at 3p25.1, is located 150 kb upstream of MBNL1, which encodes a factor that regulates splicing transitions occurring shortly after birth. The second SNP, rs573872 (OR = 1.41; P = 4.3 × 10(-12)), maps to an intergenic region at 3p25.2 approximately 1.3 Mb downstream of MBNL1. The third SNP, rs29784 (OR = 1.42; P = 1.5 × 10(-15)) at 5q35.2, is 64 kb downstream of NKX2-5, which is involved in development of cardiac muscle tissue and embryonic gut development.