Human papillomaviruses (HPVs) are associated with a spectrum of different diseases in humans, including common warts and genital warts. Of more serious concern is the connection between certain HPV types and some malignancies, particularly cervical and anal cancer. DNA from HPV-16 and HPV-18, two types frequently found in cervical cancer tissue, can immortalize cells in laboratory cultures, unlike DNA from HPV types associated with benign genital lesions. Although it is unclear how high-risk HPV types cause cancer, studies indicate that malignant transformation involves the viral E6 and E7 gene products, which may exert their effect by interfering with the cellular proteins that regulate cell growth. The vast majority of those infected do not develop malignancies, indicating that HPV infection alone is not enough to cause cancer. Cofactors such as cigarette smoking, may be required before neoplasia can occur. The potential seriousness of HPV infections is suggested by the observations that the number of genital HPV infections diagnosed is increasing and that cervical cancer is the second leading cause of cancer deaths in women throughout the world.