Bowel volume loss during anus-preserving surgery (APS) may result in low anterior resection syndrome (LARS). We conducted this prospective observational cohort study to measure the incidence of LARS after surgery and evaluate the relationship between bowel volume loss and bowel function.Patients with R0 resectable rectal cancer who consented to several bowel function surveys through telephone interviews after the operation were included. Enrolled patients underwent standard APS for rectal cancer, and three length indexes, viz. length of excised bowel, length of the distal margin and length of the proximal margin (LPM) of fresh bowel specimens, were measured in vitro.The three measured variables of the specimens showed a positively skewed distribution. Patient interviews revealed a trend of gradual improvement in bowel function. Univariate analyses revealed that longer LPM was associated with a significantly negative impact on bowel function at all time points. In multivariate analysis, LPM was found to be a significant risk factorstatistically significant, but its impact was not as strong as that of radiotherapy and low-middle tumour. Furthermore, there was no significant difference in the lymph node detection rate between <10-cm and ≥10-cm LPM groups.In APS for rectal cancer, bowel volume loss is an important factor causing postoperative bowel dysfunction. Controlling LPM to <10 cm may help improve postoperative bowel function.© 2023 The Authors. Published by Elsevier Ltd.