['Despite early diagnosis of prostate cancer, seminal vesicle invasion is still a common clinical scenario nowadays. The objective of this study is to evaluate clinical and pathological prognostic factors in that subgroup of patients.', 'After approval of our Ethical Committee, we selected all pT3b prostate cancer patients operated between 1987 and 2010. Neoadjuvant treatment patients were excluded. The biochemical free survival periods BFS and the period free of complementary treatment were calculated with the Kaplan Meier method. Cox regression model was used to select those variables associated with biochemical failure and the need for complementary treatment. We considered complementary treatment when radiotherapy or hormone therapy in an adjuvant or salvage scheme was required.', '101 patients were selected from 1470 procedures. Among these, 28 patients died (27,7%), 18 due to tumor, and 74 showed biochemical relapse (73,3%). The median follow up was of 4 years and 4 months. The five years BFS was 30.2% (IC 95%: 20.2-40.1), whereas the 5 year period free of complementary treatment was 16.9% (IC 95%: 8.1-25.8%). In the multivariate analysis, margin status (R) was independently and significantly associated with biochemical relapse and the need for complementary treatment. Likewise, the preoperative PSA was associated to biochemical relapse and N1 tumours were clearly associated to complementary treatment.'] Is margin status a very important prognostic factor for patients with pT3b prostate cancer?