Epidemiologic studies have reported the underrepresentation of cervical precancerous lesions caused by human papillomavirus (HPV) types 18 and 45 (HPV18/45) compared with the proportion of cervical cancers attributed to these HPV types. We investigated the timing of diagnosis of histologic cervical intraepithelial neoplasia grade 3 or worse (CIN3+) using data from the atypical squamous cells of undetermined significance–low-grade squamous intraepithelial lesion triage study (ALTS). Of the 2,725 women who underwent enrollment colposcopy, 412 of 472 (87.3%) diagnosed with histologic CIN3+ over the 2-year duration of ALTS could be assigned to a HPV type or group of types and were included in this analysis. Eighty-four percent of HPV16-positive CIN3+ were diagnosed at enrollment, compared with 57% of HPV18/45-positive CIN3+, and 58% of CIN3 positive for other carcinogenic HPV types at enrollment. In contrast, only 8% of HPV16-positive CIN3+ were diagnosed at exit, whereas 31% were HPV18/45 positive and 22% were positive for other carcinogenic types at study exit (P < 0.001). These results indicate the underrepresentation of HPV18/45 in precancers, whereas HPV16-associated CIN3+ is diagnosed much earlier. Whether the underrepresentation of 18/45 may be due to occult pathology needs further investigation. [Cancer Res 2009;69(8):3262–6]