Purpose: Recent epidemiological data indicate that a history of increased exposure to sexually transmitted diseases is associated with an increased risk of prostate cancer. Human cytomegalovirus (HCMV) is a member of the herpesvirus family, is sexually transmitted in adults and can persistently infect prostatic epithelium in nonimmunocompromised hosts. Based on increased awareness of the oncogenic potential of this virus, we decided to reexplore the issue of whether HCMV might be involved in prostate cancer pathogenesis. Materials and Methods: Paraffin embedded biopsy specimens from 22 randomly selected patients with prostatic intraepithelial neoplasia (PIN) lesions and prostatic carcinoma were analyzed by immunohistochemistry, in situ hybridization, polymerase chain reaction and DNA sequencing to detect HCMV nucleic acids and determine whether HCMV gene products were specifically associated with neoplastic cells. Results: We detected HCMV proteins and/or nucleic acids in all 22 of the 22 preneoplastic and neoplastic prostate lesions evaluated. HCMV proteins were specifically and often highly expressed in basal cell hyperplasia and PIN lesions, and to a lesser degree in carcinoma cells. Results: To our knowledge these data demonstrate for the first time the specific localization of HCMV nucleic acids and proteins in a high percent of PIN and prostate carcinoma lesions, and raise the possibility that HCMV might contribute to the natural history of prostatic cancer.