['To assess if prostate-specific antigen (PSA) nadir is an independent predictor of treatment failure and disease-free survival after high-intensity focussed ultrasound (HIFU) therapy for localised prostate cancer as defined by the new ASTRO criteria.', 'One hundred three patients after HIFU treatment (Ablatherm, EDAP, Lyon, France) for localised prostate cancer without previous hormonal therapy were evaluated retrospectively. Patients attended regular follow-up visits every 3 mo. Treatment failure was defined by the revised ASTRO criteria (PSA >or=2 ng/ml above nadir PSA, positive biopsy, if salvage treatment was administered). Patients were divided into three PSA nadir subgroups (group 1, <or=0.2 ng/ml; group 2, 0.21-1 ng/ml; group 3, >1 ng/ml). The disease-free survival rate (DFSR) was calculated by using life table methods. The log-rank test was used to compare the curves based on Kaplan-Meier models.', 'The median follow-up was 4.9 (3-8.6) yr. Mean time to PSA nadir was 6.4+/-5.1 mo. A PSA nadir of <or=0.2 ng/ml, 0.21-1 ng/ml, and >1ng/ml was reached by 64%, 22.3%, and 13.6% of patients, respectively. Treatment failure rates during follow-up were 4.5%, 30.4%, and 100%, respectively, for the three groups (p<0.001). The actuarial DFSRs at 5 yr were 95%, 55%, and 0%, respectively, for the 3 groups (p<0.001).'] Is pSA nadir a significant predictor of treatment failure after high-intensity focussed ultrasound ( HIFU ) treatment of localised prostate cancer?