Objective: To investigate the long-term utility of adjuvant therapy after radical prostatectomy (RP) for prostate cancer with seminal vesicle invasion (SVI; pT3b), as the published data are conflicting. Patients and Methods: Patients with SVI during RP and pelvic lymph node dissection at two major referral centres from 1986 to 2014 were included. Kaplan-Meier analyses and multivariable Cox regressions were used to determine if adjuvant radiotherapy (aRT) and adjuvant hormonal therapy (aHT) were predictors of biochemical recurrence (BCR), cancer-specific mortality (CSM) and overall mortality (OM). Subset analyses were performed for pN0 patients and pN+ patients. Results: Overall, 3 279 patients with prostate cancer and SVI were included with a median follow-up of 148 months. Considering the whole SVI population, 1 387 (42%) received no adjuvant therapy, 1 179 (36%) received aHT, 461 (14.1%) received aRT, while 252 (7.7%) received both aHT and aRT. The 10-year BCR, CSM, and OM rates were 64%, 14%, and 27%, respectively. In the overall population, aRT and aHT were predictors of BCR, CSM and OM (all P <0.04). When only pT3bN0 patients were considered, aHT was a significant multivariate predictor of BCR [hazard ratio (HR) 0.50, P <0.001), CSM (HR 0.62, P = 0.01) and OM (HR 0.75, P = 0.004). Conversely, aRT was not associated with survival outcomes (all P > 0.05). When only the subgroup pT3bN+ was considered, the use of aRT was related to an improvement in CSM (HR 0.65, P = 0.03) and OM (HR 0.78, P = 0.03). Conclusions: aHT + aRT seems to be effective in pT3b patients. However, when stratified according to the presence of nodal metastases, aHT remains effective only in the node-negative subgroup, while aRT remains effective only in the node-positive subgroup. Further data including prospective trials are warranted to study the utility of adjuvant therapies in this setting. © 2016 BJU International.