Purpose <p>Rectal dilatation is common in anorectal malformations (ARMs), predisposing patients to postoperative defecation dysfunction. However, studies of diagnostic criteria and risk factors are lacking.</p> Methods <p>This retrospective study enrolled 100 male patients with intermediate/high-type ARMs who underwent high-pressure distal colostogram (HPC) prior to anoplasty. The recto-sacral index (RSI), defined as the maximum diameter of the rectal pouch divided by the height of the second sacral vertebra, was developed and measured. Patients were categorized based on intraoperative and pathological findings of rectal dilatation. Demographic and fistula-related factors were compared between groups. Receiver operating characteristic (ROC) curve analysis was used to determine the optimal RSI cutoff for diagnosing rectal dilatation.</p> Results <p>The incidence of rectal dilatation and the recto-sacral index (RSI) varied significantly according to ARMs type (<i>P</i> = 0.001 and <i>P</i> &lt; 0.001, respectively), being highest in rectovesical and rectoprostatic (high-type) fistulas. A higher RSI was also associated with the absence of meconium per urethra (<i>P</i> = 0.029) and longer fistula length (<i>P</i> = 0.039). The RSI was significantly larger in the dilatation group (2.54 ± 0.09) than in the non-dilatation group (1.89 ± 0.05, <i>P</i> &lt; 0.001). ROC analysis identified an RSI cutoff of &gt; 2.34 for diagnosing rectal dilatation, with an area under the curve of 0.841 (95% CI: 0.762–0.921, <i>P</i> &lt; 0.001).</p> Conclusion <p>Rectovesical/prostatic fistulas, longer fistula, and no history of meconium per urethra may be predictive risk factors for rectal dilatation in male patients with ARMs. The RSI &gt; 2.34&#xa0;serves as a reliable and objective imaging-based criterion for preoperative identification of rectal dilatation.</p>