Human papillomaviruses (HPVs) have been recognized as the etiological agent of warts, and they may also be associated with many cancers. HPV-18 is very common both in genital papillomas and in large bowel cancer. The relation between HPV-18 infection and natural course of colorectal cancer has not been fully defined. In this study, normal mucosa and colorectal cancer tissue were evaluated for the presence of HPV gene to determine whether or not HPV was involved in the development of colon neoplasm. The DNA extracted from colon tissue was screened for HPV by polymerase chain reaction (PCR) to amplify the viral gene fragment. These PCR products were digested with restriction enzyme, and Southern blotting was then performed to confirm the existence of HPV-18. The nucleotide sequence related to HPV-18 DNA was detected in 53% (10/19) of the normal mucosa specimens and in 84% (16/19) of the colorectal cancer specimens. The correlation between cancer samples and positive rate of HPV-PCR was statistically significant by chi-square test (p < 0.01). These data indicate that HPV-18 can infect the normal mucosa of the colon, and that this infection may be a risk factor for the development of colorectal cancer. The presence of HPV-18 DNA in patients with colorectal cancer suggests that the pathogenesis of colorectal cancer includes viral involvement.