['The aim of this study was to study nitric oxide synthase (NOS) immunohistochemistry in the pyloric muscle and establish the role of nitric oxide in pyloric stenosis.', 'Pyloric muscle biopsy specimens were obtained from 20 patients with pyloric stenosis during pyloromyotomy. Ten control specimens without pyloric disease were obtained from autopsy performed less than 4 hours after death on age-matched babies who died of other causes. Tissues were fixed in 4% paraformaldehyde immediately. A monoclonal antibody against the neuronal form of NOS (bNOS) was used for immunohistochemistry.', 'Immunohistochemistry showed activity of bNOS in the control specimens and some pyloric stenosis specimens. This shows that NOS is present in the pylorus in normal cases as well as in a few cases of pyloric stenosis.'] Is nitric oxide synthase absent in only a subset of cases of pyloric stenosis?