Objective: Patients with asymptomatic haemorrhoids are known to be less tolerant of radiation doses lower than known tolerance doses. In the present study, the authors sought to identify the risk factors of acute haemorrhoid aggravation after whole pelvic radiotherapy (WPRT). Methods: The records of 33 patients with cervical, rectal or prostate cancer with asymptomatic haemorrhoids, which were confirmed by colonoscopy before the start of radiotherapy (RT), were reviewed. Acute anal symptoms, such as anal pain and bleeding, were observed up to 1 month after RT completion. Dosimetric and patient factors were analysed, and subgroup analyses were performed.Results: The median induction dose for acute anal symptoms was 34.1 Gy (range, 28.8-50.4 Gy). Postoperative treatment intent showed more acute anal toxicity of patient factors (p 5 0.04). In subgroup analysis, post-operative treatment intent and concurrent chemoradiotherapy were found to be related to acute anal symptoms (p , 0.01). Of the dosimetric factors, V 10