Late onset of radiation-induced haemorrhagic cystitis (RHC) after radiation therapy (RT) for prostate cancer (PCa) may present or evolve severely, requiring hospitalization with invasive interventions. In the present study, we have analysed the prevalence and risk factors associated with the onset of RHC. From January 2002 to May 2017, 1421 patients undertook RT for PCa as a primary, adjuvant, or salvage treatment option. RHC presented in 5.6% (n = 80) of the patients; the diagnosis was based on clinical and endoscopic characteristics. Variables in observation included patients, tumours, and RT-dosimetry characteristics. Patients with a previous history of bladder cancer were excluded. Univariate (Student t/Chi square) and uni-/multivariate Cox regression analysis were performed; the events and time-points were hospitalization and time-to-event, respectively. There were 80 patients with a mean age at RT of 70.1 years (SD 6.4), mean time lag to RHC of 43.9 months (SD 37.5). Median Emergency attendance was two and three times for patients without/with hospitalization, respectively. There were in total 64 admissions with invasive treatment required in 26/36 (72.2%) of the patients hospitalised, including transurethral fulguration in 22 and radical cystectomy in 5. Patients at higher risk of hospitalization were those undertaking antiplatelet/anticoagulant treatment (HR:3.30; CI 95%:1.53–3.30; p = 0.002) and those treated with salvage RT with higher bladder volume receiving >70 Gy (bladder V70) (HR:1.03; CI 95%:1.01–1.05; p = 0.027). At receiving operating characteristic analysis, the cutoff for bladder V70 was 29%. Nearly half of patients presenting RHC may require invasive treatment including cystectomy. Risk factors associated with hospitalization are patients undertaking antiplatelet/coagulant treatment and bladder V70 > 29% in salvage RT patients.