Objectives: The Roach formula can be used to calculate the risk of pelvic lymph node involvement in patients with prostate cancer. This study aimed to use the Roach formula to further differentiate high-risk patients at risk of treatment failure after intensity-modulated radiotherapy (IMRT), and to identify factors associated with biochemical failure-free survival (bFFS). Methods: Records of consecutive patients with biopsy-proven localised prostate cancer (T1-4 N0M0) who underwent prostate-only IMRT between February 2006 and August 2011 were retrospectively reviewed. Neoadjuvant and concomitant androgen deprivation therapy (ADT) was given to intermediate-and high-risk patients, whereas adjuvant ADT was given to high-risk patients for 2 to 3 years if they could afford this selffinanced item. Patients were divided into three groups of lymph node involvement based on the Roach formula (≤15%, >15-35%, and >35% risk) and their bFFS were compared. Factors associated with bFFS were identified using univariate and multivariate analyses. Results: The median follow-up duration of 144 patients was 55.8 months. According to the National Institute for Health and Care Excellence classification, 6%, 30%, and 64% of patients were stratified as low, intermediate, and high risk, respectively. According to the Roach formula, 35%, 28%, and 37% of patients were stratified as low, (≤15%), intermediate (>15-35%), and high (>35%) risk of lymph node involvement, respectively. Biochemical failure occurred in 23 patients. The median bFFS was 48.5 months. The 5-year bFFS in the three groups of ≤15%, >15-35%, and >35% risk of lymph node involvement based on the Roach formula were 100%, 87.7%, and 75.4%, respectively (p = 0.003). In multivariate analysis, significant factors associated with better bFFS were patient age of >75 years, pretreatment serum prostate-specific antigen (PSA) of ≤20 ng/ml, undetectable serum PSA after IMRT, and longer duration of adjuvant ADT. Conclusion: The Roach formula can further differentiate patients at higher risk (>15-35% and >35%) of lymph node involvement to receive more intensified IMRT and closer monitoring to improve their bFFS.