BACKGROUND: A narrow pelvis makes laparoscopic rectal resection difficult. This study aimed to evaluate whether a simple measurement on computed tomography can predict procedural difficulty. METHODS: A total of 62 patients with low rectal cancer underwent conventional laparoscopic low anterior resection. The interischial spine (IS) distance (ie, the distance between the ischial spines) was measured on an axial computed tomography slice. The operative time, blood loss, and time from the insertion of linear staplers to completion of clamping on the distal end of the rectum (clamp time) were compared between patients with narrow or wide pelvises. RESULTS: Overall, 42 men and 20 women with low rectal cancer were assessed. The mean tumor size was 34.5 mm. Total or tumor-specific mesorectal excisions were performed in all cases; high ligation and resection of the inferior mesenteric arteries were carried out in 92% of patients. The mean operative time and blood loss were 206 minutes and 15 mL, respectively. Four patients (6.5%) experienced postoperative complications, including 2 anastomotic leaks (3.2%). The mean IS distance was 93.3 mm. In simple linear regression analysis, a shorter IS distance correlated with a longer operative time (R2=0.08, P=0.030) and the clamp time (R2=0.07, P=0.046). Using a receiver operating characteristic curve, a narrow pelvis was defined as an IS distance <94.7 mm. Multivariate regression analysis revealed that an IS distance <94.7 mm (odds ratio: 3.51; P=0.04) was independently associated with a longer clamp time. CONCLUSIONS: The IS distance is a simple and useful measurement for predicting the difficulty of laparoscopic low anterior resection.