Conformal photon and proton therapy plans for prostate cancer have been compared in an attempt to quantify the potential advantages of using protons. Two X-ray plans (3-field, 6-field) and a 2-field proton plan were made and compared for each of 20 T3 prostate patients with the aid of the 3D planning system VOXELPLAN. Dose distributions were analysed in terms of dose-volume histograms (DVH). Tumour control probability (TCP) and normal tissue complication probability (NTCP) were computed using our own and the Lyman-Kutcher-Burman models, respectively. The study shows that on average the proton technique results in the best dose distribution, giving the lowest rectal complication probability, and also that the 3-field X-ray technique is more effective than the 6-field X-ray technique in sparing the rectum. At 5% rectal NTCP, the predicted proton average TCP for the 20 patients is 2% (in absolute terms) greater than that obtained using 3-field X-ray therapy. For 7 of the patients the gain in TCP is more than 3%. For the same rectal NTCP as the 3-field X-ray plan with a 64 Gy mean target dose, the use of protons increases the TCP by 2% on average, but for 5 of the patients the increases are greater than 4%. The result is in general positive towards the use of protons but a few patients do not benefit from it and this indicates the importance of patient selection for maximum clinical benefit.