The etiological role of the gene for neuronal nitric oxide synthase (NOS1) in infantile pyloric stenosis (PS) was investigated by analysis of two intragenic polymorphisms (NOS1a and NOS1b) in 27 families. There was significant overall transmission disequilibrium between PS and NOS1a (P = .006). Consideration of each allele independently revealed a highly significant tendency for allele 7 (210 bp) to be preferentially transmitted to the affected offspring (P = .0006). These observations suggest that NOS1 is a susceptibility locus for PS.