Cervical intraepithelial neoplasia grade 3 (CIN 3) is the best proxy in research and screening for invasive cancer risk. Yet the timing of CIN 3 development is uncertain because of measurement errors integral to its diagnosis. We were interested in estimating the proportions of prevalent vs incident CIN 3 within 2 years of finding a minor cytologic abnormality. We estimate that only 17 (2.8%) of 613 CIN 3 cases diagnosed during the 2-year duration of the atypical squamous cells of undetermined significance (ASCUS) and low-grade squamous intraepithelial lesion (LSIL) triage study (ALTS) were incident CIN 3 following an incident human papillomavirus (HPV) infection that persisted until the CIN 3 diagnosis was made. Using prevalent high-grade cytology as a marker of prevalent CIN 3, we estimated that another approximately 23% of CIN 3 cases were incident CIN 3 following a prevalently detected HPV infection that persisted until the CIN 3 diagnosis was made. We concluded that most CIN 3 cases diagnosed within the 2-year time frame were prevalent cases, and most incident CIN 3 cases followed a prevalently detected HPV infection.