An issue relating to uniformity in pathological staging of prostate cancer that has received relatively minimal attention is that of seminal vesicle invasion. Several studies define tumor in the per-seminal vesicle soft tissue as seminal vesicle invasion, while others equate seminal vesicle invasion with tumor invading the muscular wall of the seminal vesicle. There are also controversies regarding the prognostic significance of seminal vesicle invasion compared to capsular penetration, and whether seminal vesicle invasion is a predictor of poor prognosis independent of tumor volume and grade. We evaluated 115 cases of established capsular penetration, 16 of peri-seminal vesicle invasion and 45 of seminal vesicle invasion in patients without lymph node metastases. Patients with seminal vesicle invasion had a significantly worse prognosis than those with capsular penetration; peri-seminal vesicle invasion was associated with an intermediate risk of progression. Gleason grade, surgical margins and seminal vesicle invasion were all independent predictors of progression in a multivariate analysis, whereas tumor volume was not. In patients with seminal vesicle invasion there was a trend for surgical margins and Gleason grade to predict progression; with tumor volume there was none. Our study demonstrates that the definition of seminal vesicle invasion should be restricted to tumors showing infiltration into the muscular wall of the seminal vesicle. Our study further demonstrates that when assessing the ability of new prognostic variables to predict prognosis of patients with seminal vesicle invasion, margins of resection and Gleason grade should be considered.