Salvage radiotherapy after radical prostatectomy 733 RT, a standardized definition of biochemical failure after RP does not exist. Most investigators have suggested a PSA level of 0.2 ng/ml [3] or 0.4 ng/ml [19] as a PSA cut-off point for biochemical failure. While the PSA level prior to salvage RT is considered to be a predictor of the outcome of salvage RT and overall survival, the reported preradiotherapy PSA cut-off points for successful treatment range widely from 0.2 ng/ml to 2.5 ng/ml [6, 8, 10]. In our study, the difference between the overall bDFS rates of patients with a preradiotherapy PSA level > 0.2 ng/ml and those with a level < 0.2 ng/ml was significant. Among patients with LI, 3 of the 17 patients who received salvage RT before the preradiotherapy PSA level reached 0.21 ng/ml did not experience biochemical failure follow- ing salvage RT. Meanwhile, 12 of the other 14 patients with LI who received salvage RT after their PSA level had risen above 0.2 ng/ml experienced biochemical failure. These findings suggest that for high-risk patients, especially those with LI, a PSA level of >0.2 ng/ml may be too late for the initiation of salvage RT, and earlier RT before the PSA level reaches 0.2 ng/ml seems to be advisable. Geinitz et al. reported that distant metastases were more common in patients with a postradiotherapy PSA nadir >0.05 ng/ml [20]. We investigated the association between the prognostic factors and the postradiotherapy PSA nadir level and examined the outcome of patients with a postra- diotherapy PSA nadir >0.05 ng/ml. We identified several factors such as LI, a negative surgical margin and the preradiotherapy PSA level as being significantly associated with a high postradiotherapy PSA nadir of >0.05 ng/mL using a logistic regression analysis. These factors were the same as the predictors of biochemical failure following salvage RT. The overall 5-year bDFS rate of patients with a postradiotherapy PSA nadir >0.05 ng/mL was calculated to be 10% using the Kaplan–Meier method. These facts indicate that patients with LI, a negative surgical margin and a high preradiotherapy PSA are highly likely to experience a postradiotherapy PSA nadir of >0.05 ng/ml, subsequent biochemical failure and distant metastasis. Thus, a postradiotherapy PSA nadir of >0.05 ng/ml may predict a poor outcome of salvage RT and a poor prognosis. A limitation of our study is its retrospective design and the small group setting. A randomized clinical trial is needed to define the most appropriate technique for per- forming salvage RT. Furthermore, since the methods and sensitivities of PSA measurements have changed, studies with a unified protocol are needed. In conclusion, the pathological features affected the outcome of salvage RT following RP. 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