Background and Objective The cardinal diagnostic sign of congenital aganglionic megacolon, or Hirschsprung's disease (HD), is an aganglionic segment of the distal colon or rectum. To determine the surgical planning of a radiological transition zone (TZ) in HD, this study investigated the association between a radiological TZ and the bowel resection length. Methods A prospective observational study was conducted in children (n = 192) with suspected HD determined by radiological TZ on contrast barium enema, and who underwent pull-through operations. The bowel resection length was ae<yen>10 cm above the proximal radiological TZ levels and confirmed by intraoperative frozen sections. In the contrast enema, the presence and level of a radiological TZ were recorded. Correlation of the TZ features with ganglion cells assessed by immunostaining of neuronal nuclei (NeuN) and the odds ratio were calculated. Results The sensitivity and specificity for diagnosing HD by the presence of a radiological TZ were 86.9 and 92.1%, respectively; Youden's index was 79.0%. The positive and negative predictive values were 91.7 and 87.6%. The kappa value indicating an association between TZ and HD was 0.776 (P < 0.05). The correlation rate between a radiological TZ and the pathological results was 88.5% in the rectosigmoid colon and 44.4% in the descending colon, and was higher in children older than 3 months (85.3%) than in infants (69.0%). Conclusion A preoperatively determined radiological TZ has potential value to identify the length of resected bowel in patients with HD, and it also has a high predictive value for diagnosis of HD.