BACKGROUND: More information is needed about time between sexual initiation and human papillomavirus (HPV) infection and development of cervical precancer. METHODS: The objectives were to investigate the time between first sexual activity and detection of first cervical HPV infection or development of first cervical intraepithelial neoplasia (CIN), and associated factors in women from the double-blind, multinational, 4-year PATRICIA trial. PATRICIA enroled women aged 15-25 years with no more than 6 lifetime sexual partners. Women were randomized 1:1 to the HPV-16/18 AS04-adjuvanted vaccine or to control, but only women from the control arm who began sexual intercourse during the study or within 6 months before enrolment, and had no HPV infection detected before the recorded date of their first sexual intercourse, were included in the present analysis. The time between onset of sexual activity and detection of the first cervical HPV infection or development of the first CIN lesion was analyzed using Kaplan-Meier and univariate and multivariable Cox proportional-hazards models. RESULTS: A total of 9337 women were enroled in the control arm of PATRICIA of whom 982 fulfilled the required inclusion criteria for analysis. A cumulative total of 28%, 44%, and 62% of the subjects had HPV infection within 12, 24, and 48 months, respectively. The overall incidence rate was 27.08 per 100 person-years. The most common oncogenic types associated with 6-month persistent infection were HPV-16 (incidence rate: 2.74 per 100 person-years), HPV-51 (2.70), HPV-52 (1.66), HPV-66 (1.14), and HPV-18 (1.09). Increased infection risk was associated with more lifetime sexual partners, being single, Chlamydia trachomatis history, and duration of hormone use. CIN1+ and CIN2+ lesions were most commonly associated with HPV-16, with an overall incidence rate of 1.87 and 1.07 per 100 person-years, respectively. Previous cervical HPV infection was most strongly associated with CIN development. CONCLUSIONS: More than 25% of women were infected with HPV within 1 year of beginning sexual activity. Without underestimating the value of vaccination at older ages, our findings emphasize its importance before sexual initiation. TRIAL REGISTRATION: clinicaltrials.gov: NCT00122681 .