We reported localized amyloidosis involving seminal vesicles and vasa deferentia, which was found in two patients with prostatic adenocarcinoma. A 60-yr-old (Case 1) and a 59-yr-old (Case 2) man came to our hospital with elevation of serum prostate-specific antigen (PSA) and biopsy proven carcinoma, respectively. MRI revealed multiple irregular foci of low signal intensity in the prostates as well as in both seminal vesicles and vasa deferentia on T2-weighted imaging, suggesting prostatic carcinoma with extension to both seminal vesicles and vasa deferentia in both cases. Under the clinical diagnosis of stage III prostatic adenocarcinoma, a radical prostatectomy was performed in both patients. Microscopically, Gleason score 7 adenocarcinoma was observed in both patients. In addition, isolated amyloidosis of both seminal vesicles and vasa deferentia was found without carcinoma involvement. Localized amyloidosis in the seminal vesicles, which is considered as senile process, has been occasionally reported in the autopsy and in the surgical specimens. Amyloid deposition in the vas deferens has also been reported in the literature, however, the deposition mimicking extension of carcinoma has not been reported. In this report, two cases of isolated amyloidosis of the seminal vesicles and vasa deferentia are described with electron microscopic study and literature review.