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To explore the association between the prostate-specific antigen (PSA) nadir after transrectal high-intensity focused ultrasound (HIFU) therapy for organ-confined prostate cancer and subsequent treatment failure, as defined by the presence of residual disease at biopsy 6 months after treatment.Between January 1999 and January 2005, 115 patients in a Japanese hospital were treated using a transrectal HIFU system (Sonablate™, Focus Surgery, IN, USA) for presumed localized adenocarcinoma of the prostate. All treatments were primary and none of the patients had received hormone therapy. The PSA level was measured at 2-monthly intervals and all patients had a transrectal prostate biopsy taken at 6 months. Multiple logistic regression was used to examine the relationship between PSA nadir and treatment failure, as defined by the presence of disease at biopsy.The PSA nadir was strongly associated with treatment failure (P < 0.001). Patients with a PSA nadir of 0.0–0.2 ng/mL had a treatment failure rate of only 11% (four of 36), compared to 46% (17 of 37) in patients with a PSA nadir of 0.21–1.00 ng/mL and 48% (20 of 42) with a PSA nadir of >1.0 ng/mL. In addition, the PSA nadir was strongly associated with both preoperative PSA level and residual prostate volume.There is a clear and intuitive association between the PSA nadir and the risk of treatment failure after HIFU. These data can be used to predict the risk of residual disease in patients with prostate cancer undergoing HIFU therapy. They can also be used to inform where the target PSA nadir should be set for this novel therapy.