The purpose of this nested case-control study was to estimate the risk of SIL development among a cohort of women providing cervical samples as part of their family planning visit at baseline in 1991–1992. All women had normal cervical cytology (N = 2905) at baseline and provided a cervical sample for subsequent HPV typing. Among this cohort, 426 women developed SIL (22 HSIL and 404 LSIL), 619 developed atypia, and 1860 remained cytologically normal. Two controls per case were sampled from those who remained normal. PCR-based methods with L1 consensus primers were used to assess high-risk HPV positivity. Having an oncogenic HPV type at baseline was associated with an almost fourfold increased risk of HSIL development (relative risk (RR) = 3.8; 95% CI, 1.5–9.0) and a 70% increased risk of LSIL development (RR = 1.7; 95% CI, 1.2–2.3%). The association between HPV positivity and SIL development was strongest in the first year of follow-up (RR = 9.2 for HSIL and 2.5 for LSIL development). The decline in HPV-associated SIL risk may be a function of having only one measure of HPV positivity (at baseline).