The aims of this study were to investigate whether prostate cancer patients want to be involved in the choice of the radiation dose, and which patients want to be involved. This prospective study involved 150 patients with localized prostate cancer treated with three-dimensional conformal radiotherapy. A decision aid was used to explain the effects of two alternative radiation doses (70 and 74 Gy) in terms of cure and side effects. Patients were then asked whether they wanted to choose their treatment (accept choice), or leave the decision to the physician (decline choice). The treatment preference was carried out. Even in this older population (mean age, 70 years), most patients (79%) accepted the option to choose. A lower score on the designations Pre-existent bowel morbidity, Anxiety, Depression, Hopelessness and a higher score on Autonomy and Numeracy were associated with an increase in choice acceptance, of which only Hopelessness held up in multiple regression (p < 0.03). The uninformed participation preference at baseline was not significantly related to choice acceptance (p = 0.10). Uninformed participation preference does not predict choice behavior. However, once the decision aid is provided, most patients want to choose their treatment. It should, therefore, be considered to inform patients first and ask participation preferences afterwards.