PURPOSE: We determined whether prostate specific antigen criteria after focal high intensity focused ultrasound to treat prostate cancer could diagnose treatment failure. MATERIALS AND METHODS: A total of 598 patients in a prospectively maintained national database underwent focal high intensity focused ultrasound with a Sonablate® 500 device from March 2007 to November 2016. Followup consisted of 3-month clinic visits and prostate specific antigen testing in year 1 with prostate specific antigen measurement every 6 to 12 months and multiparametric magnetic resonance imaging with biopsy for magnetic resonance imaging suspicious for recurrence. Treatment failure was considered any secondary treatment, tumor recurrence with Gleason 3 + 4 or greater disease on prostate biopsy without further treatment or metastasis and/or prostate cancer related mortality. To diagnose failure we evaluated a series of nadir + x thresholds with x values of 0.1 to 2.0 ng/ml. RESULTS: Median patient age was 65 years (IQR 60-71) and the median Gleason score was 7 (range 6-9). Gleason 3 + 4 or greater disease was present in 80% of cases. Tumors were radiologically staged as T1c-T2c in 522 of the 596 patients (88%) and as and T3a/b in 74 (12.4%). Baseline median prostate specific antigen was 7.80 ng/ml (IQR 5.96-10.45) in failed cases and 6.77 ng/ml (IQR 2.65-9.71) in cases without failure. Optimal performance according to the Youden index to indicate the most appropriate nadir + x at all analyzed time points at 3-month intervals showed that nadir + 1.0 ng/ml would have 27.3% to 100% sensitivity and 39.4% to 85.6% specificity depending on the time of evaluation in the first 3 years. Nadir + 1.5 ng/ml showed 18.2% to 100% sensitivity and 60.6% to 91.8% specificity with nadir + 2.0 ng/ml leading to similar sensitivity and specificity ranges. Nadir + 1.0 ng/ml at 12 months and nadir + 1.5 ng/ml at 24 and 36 months had 100% sensitivity and 96.1% to 100% negative predictive value. CONCLUSIONS: Following focal high intensity focused ultrasound a prostate specific antigen nadir of 1.0 ng/ml at 12 months and 1.5 ng/ml at 24 to 36 months might be used to triage men requiring magnetic resonance imaging and biopsy. These data need prospective validation.