[en] Aim: 1) To determine a nadir PSA value following external beam radiation therapy that predicted a high probability of freedom from a subsequently rising PSA. 2) To determine the range of rates of PSA rise for those who do fail. Method and Materials: Kaplan-Meier analysis was performed on 172 men with T1-2 disease treated by external beam irradiation alone between 1988 and 1992. All men had pre- and post-treatment serum PSA evaluation and all had at least 2 years of follow-up to ensure the nadir was reached and evaluation beyond this available. Failure was defined as two successive post-treatment rises in serum PSA of greater than 10%. A closer examination was also made of 126 patients relapsing after irradiation alone. PSA doubling times were calculated using an exponential model. Results: Overall 4 year biochemical disease-free survival (BDFS) was 55% (median 33 months). Those whose PSA declined to undetectable levels (≤0.5ng/ml) had a 4 year BDFS of 94%. For those who achieved a nadir PSA of ≤1.0ng/ml (n=121) it was 66%, for 1-2 it was 49% (n=27), for >2 it was 0% (n=14). For those whose serum PSA nadir was 1.0ng/ml pretreatment prognostic factors such as initial PSA and Gleason grade still predicted outcome (88% 4 year BDFS when the initial PSA was ≤4.0ng/ml, 63% for 4-10, and 44% for >10%; 82% for Gleason (1-2(5)), 69% for Gleason (3(5)), and 25% for Gleason (4-5(5))). In a multivariate analysis PSA nadir of ≤1.0ng/ml did not predict failure independently of initial PSA or Gleason grade. An undetectable nadir PSA, however, did (p=<0.001). The PSA doubling times on relapse exhibited a broad spectrum of values. For men with Gleason 1-2 tumors the median was 18.8 months, for Gleason 3 it was 11.1 months, and for Gleason 4-5 9.6 months. There were significant differences between the low and high grade median values (p=0.03) and the medium and high grade groups (p=0.04). Conclusions: 1) Only those who achieve a nadir PSA value of ≤0.5ng/ml following external beam radiation therapy stand a high chance of lasting disease control. 2) The PSA doubling times on relapse are similar to those seen after radical surgery. They reflect the initial Gleason grade and thus the intrinsic biologic potential of the tumor. The data does not support the notion of radiation-induced tumor acceleration