Antropyloric muscle thickness at US in infants: what is normal? The authors reviewed the ultrasonographic (US) images and medical records of 145 consecutive infants who were seen for evaluation of the upper gastrointestinal tract because of chronic vomiting and/or regurgitation. At US, the antropyloric muscle of each patient was measured in the midlongitudinal plane. On the basis of this measurement, the patients were divided into the following categories: group 1 (1-2 mm; 99 patients), group 2 (greater than or equal to 3 mm; 40 patients), and group 3 (2- less than 3 mm; six patients). Patients in group 1 were considered to have normal antropyloric muscle thickness, those in group 2 had abnormal thickness, and those in group 3 had muscle thickness that was not definitely normal or abnormal. The final clinical diagnoses for all of the infants in the three groups confirmed the authors' initial impressions that antropyloric muscle thickness of less than 2 mm was anatomically normal, muscle measuring 3 mm or greater was abnormal and diagnostic for pyloric stenosis, and muscle from 2 to less than 3 mm was abnormal but not specifically diagnostic for pyloric stenosis. Two of the six patients in group 3 eventually were diagnosed as having pyloric stenosis; thus, the authors believe that only those patients with antropyloric muscle less than 2 mm thick should be considered unequivocably normal. 