This Review focuses on the adverse effects of radical radiotherapy for localized prostate cancer. The adverse effects are described in the context of alternative treatment modalities. First, we consider the methodological issues that make comparison between the different treatment modalities problematic. Such issues include differences in baseline levels of urinary, bowel and sexual dysfunction, the importance of using patient-reported outcomes and the distinction between actuarial and prevalence rates of treatment-related toxic effects. Second, we describe the pattern of adverse effects that occur over time after radiotherapy. Here, we focus on evidence for a beneficial effect of radiotherapy on some urinary symptoms, and the controversy regarding the risk of secondary malignancy. Third, predictors of radiation toxicity are discussed. Accurate prediction of radiotherapy toxicity would be an invaluable tool for treatment individualization. It is noteworthy that the data on the adverse effects of prostate radiotherapy necessarily relate to treatment as it was delivered in the past. It is likely that recent technical advances, such as intensity modulation and image guidance, will further improve the toxicity profile of prostate radiotherapy.