An important question is the ultimate (i.e. with "infinite" time of follow-up) local tumor regrowth rate for patients treated for specified stages of clinically localized prostatic cancer. Using combined data from patients irradiated at Stanford University from 1956 through 1983, and patients irradiated at the Massachusetts General Hospital from 1973 through 1979, we approached this question by an evaluation of the cumulative frequency of local tumor regrowth. These curves, relating cumulative incidence of local regrowth against time, approached an asymptote. These results indicate that following 6,800 to 7,500 cGy external beam irradiation, permanent local control of cancer of the prostate occurs in 85% to 90% of patients with stage T2 (B) tumors and in 60% to 70% of patients with stage T3 and T4 (C) tumors. Further analysis of the patients with T3 and T4 tumors may identify subgroups in whom more aggressive initial therapy is indicated.