The objective is to investigate preoperative body mass index (BMI) in patients receiving beyond total mesorectal excision (bTME) surgery. The primary end point is length of postoperative stay. Secondary end points are length of intensive care stay, postoperative morbidity and overall survival. BMI is the most commonly used anthropometric measurement of nutrition and studies have shown that overweight and obese patients can have improved surgical outcomes. Patients who underwent a bTME operation for locally advanced or recurrent rectal cancer were put into three BMI (kg/m2) groups of normal weight (18.5–24.9), overweight (25–29.9) and obese (≥ 30) for analysis. Included are 220 consecutive patients from a single centre. The overall length of stay, in days ± standard deviation (range), for normal weight, overweight and obese patients was 21.14 ± 16.4 (6–99), 15.24 ± 4.3 (7–32) and 19.10 ± 9.8 (8–62) respectively (p = 0.002). The mean ICU length of stay was 5.40 ± 9.1 (1–69), 3.37 ± 2.4 (0–19) and 3.60 ± 2.4 (1–14), respectively (p = 0.030). There was no significant difference between the three groups in terms of postoperative morbidity or overall survival. Patients with a normal weight BMI in this cohort have a significantly longer length of stay in ICU and in hospital than overweight or obese patients. This is seen with no significant difference in morbidity or overall survival.