Objective. The Guanacaste study ("Guanacaste Project," or GP), was designed to investigate the role of human papillomavirus (HPV) infection and its cofactors in the development of cervical neoplasia and to evaluate new cervical cancer screening technologies. The follow-up phase of the GP was designed to study why a small proportion of women infected with HPV develop cervical intraepithelial neoplasia grade 2 (CIN 2), CIN 3, or cancer (these three together are globally referred to as ≥ CIN 2, that is, CIN 2 or worse). The purpose of this article is to describe this prospective study in detail and to present the preliminary findings regarding the incidence of cervical neoplasia. Methods. A cohort of 10 049 randomly selected women from 18 to 97 years old from Guanacaste, a province in northwestern Costa Rica, was intensively screened in 1993-1994 and then followed up for seven years after being enrolled. A questionnaire for demographic and risk factors was administered, and a pelvic examination was performed on sexually active women at each follow-up visit in order to obtain samples for screening tests and for research purposes. The final diagnosis given at the end of the enrollment phase categorized women into several groups according to the perceived risk of their developing either high-grade precursors of cancer or cancer. These groups were followed up at different intervals according to the risk of developing ≥ CIN 2. The most active follow-up (every 6-12 months) was concentrated on the women most likely to develop ≥ CIN 2, based on cytology (n = 492). The remainder of the cohort was followed either annually (n = 2 574) or after five to seven years of passive follow-up (n = 3 926). All women with possibly severe lesions detected by any technique were referred to colposcopy for further evaluation and treatment, and they were also censored from the study. Lesions ≥ CIN 2 served as both the censoring outcome and our surrogate for cancer risk. Results. Participation during follow-up was high (near 90%). Suspected ≥ CIN 2 by any screening technique censored 4.6% of women. Most of the women censored because of suspected ≥ CIN 2 came from the large group perceived at entry as being at low risk of developing ≥ CIN 2, but the greatest rates of progression to ≥ CIN 2 were observed among women perceived at entry to be at highest risk of ≥ CIN 2, based on their cytology, virology, or sexual behavior. Conclusions. The GP is the largest population-based longitudinal cohort for the study of HPV and cervical neoplasia in the world, and its results will hopefully let us soon plan future worldwide prevention strategies. Research projects such as this one require the long-term com-mitment of a large multidisciplinary team and ample financial resources. The intensive effort and expertise applied in all aspects of this study were key factors in its success as a model of cooperative, interdisciplinary cancer research in Latin America. Quality control played an important role at all times during the study and made it possible to adapt new diagnostic and screening technology to Guanacaste. The systematic follow-up of a population-based group of close to 10 000 women in Guanacaste should permit careful, time-dependent evaluation of factors postulated to be linked to the development of cervical cancer as well as the evaluation of clinical markers of disease progression. The study results that have already been published have validated sensitive screening techniques and have also promoted the use of more affordable screening techniques in resource-poor, developing countries. The GP has also contributed to building knowledge for the search for vaccines against HPV as part of the effort to develop an effective tool to reduce the incidence and mortality of cervical cancer worldwide.