A total of 1200 patients had undergone I-125 prostate brachytherapy (BXT) in our centre. We present prospective outcome data for the first 400 treated patients. Data were analysed from a prospective database of 400 consecutive patients treated with permanent prostate BXT between March 1999 and December 2003. Patients were stratified into low (49%), intermediate (36%) and high (15%) risk as defined by the Memorial Sloan-Kettering Prognostic Index. Patients received 145 Gy BXT alone (41%), BXT with 3 months neoadjuvant androgen deprivation (NAAD) (39%), 45 Gy external beam radiotherapy (EBRT) with 110 Gy BXT (3%) or a combination of NAAD, 45 Gy EBRT and 110 Gy BXT (17%). Biochemical relapse-free survival (bRFS) and prostate-specific antigen (PSA) nadirs were analysed for treatment received in each risk group. Median follow-up was 54 months (range, 38-96 months) with a mean patient age of 63 years. Prostate cancer-specific survival was 99.5%. Twenty-eight patients (7%) experienced biochemical failure according to the 2006 Radiation Therapy Oncology Group-American Society for Therapeutic Radiology and Oncology (RTOG-ASTRO) Phoenix Consensus definition (PSA nadir plus >or=2 ng ml(-1)): nine low-, fourteen intermediate- and five high-risk patients. When stratified by treatment group for low-, intermediate- and high-risk groups, the 5-year actuarial bRFS was 98, 89 and 100% for BXT; 91, 87 and 88% for NAAD and BXT; 100, 80 and 100% for EBRT and BXT; and 100, 92 and 88% for NAAD, EBRT and BXT, respectively. Overall 4- and 5-year PSA <or=0.5 ng ml(-1) was achieved by 83 and 86%. This prospective study updates our biochemical survival results, and further demonstrates that the UK centres are able to reproduce the excellent US-reported outcomes for BXT.