In this study, we demonstrate that expression of cyclin B protein is up-regulated and persists into the upper epithelial layers in parallel with cyclin A expression in high-grade squamous intraepithelial lesions (SIL) infected with human papillomaviruses 16, 31, 33, 51, 58, 66, and 67 (n = 33). In contrast, low-grade SIL infected with human papillomaviruses 16, 18, 31, 33, 39, 51, 52, 56, 58, and 66 (n = 27) show weaker cyclin B expression confined to basal and parabasal cells despite extension of cyclin A and Ki67 expression into superficial cells. Moreover, aneusomy is present in 20% of the high-grade lesions but in none of the low-grade lesions. The persistent expression of cyclin B in high-grade SIL, and the restriction of aneusomy to high-grade SIL suggest that there is cell cycle progression. In combination with in vitro studies, this provides evidence that high-grade SIL lesions have undergone immortalization.