To assess the mechanisms and prognostic significance of seminal vesicle involvement (SVI) by prostatic adenocarcinoma, we analyzed 312 radical prostatectomy specimens obtained from patients with T1-T3 prostate cancer. Detailed pathological examination demonstrated three patterns of SVI. Type I involvement was direct spread along the ejaculatory duct complex into the seminal vesicles. Type II involvement was spread outside of the prostate, through the capsule, and then into the seminal vesicle. Type III involvement was characterized by the finding of isolated deposits of cancer in the seminal vesicle with no contiguous primary cancer in the prostate. We found SVI in 64 patients (21%), who have been followed for a mean of 31 months (range 1-101). A defining criterion for progression was clinically apparent local or distant recurrence or a postoperative serum prostate specific antigen (PSA) > or = 0.4 ng/ml (Hybritech). Type I SVI was found in 17 (26%), Type II in 21 (33%), and Type III in 8 (13%) cases. In 18 patients (28%), the pattern of SVI appeared to be a combination of types I and II (categorized as Type I+II). Type III (isolated metastasis) SVI was associated with significantly smaller cancers (median, 3.13 vs. 6.7 cc; p < 0.0005) and fewer positive margins (0 vs. 32%; p = 0.05) than in other types. Type II SVI, with direct extension through the capsule, was associated with a significantly higher risk of lymph node metastasis (8 vs. 33%; p < 0.05). When patients with lymph node metastases were excluded, there was a trend toward a more favorable prognosis (p = 0.09) for patients with type III SVI than with other types. Overall, patients with type III SVI had a progression-free survival rate similar to that of 83 patients with extracapsular extension without SVI. We conclude that the prognostic significance of SVI may not be uniformly ominous; instead, it may depend on the specific mechanism of involvement and the pathologic features of the primary tumor.