This study aimed to estimate the incidence and types of cervical cytopathological findings in adolescents who were treated in public health services between 1993 and 2006. This is a cohort study, with the following inclusion criteria: < 20 years of age, sexually-active, without cervical lesions upon entry into the study or sexually active < 1 year. The data were collected from 403 adolescents' medical records. Incidence density of cytopathological alterations was estimated and the actuarial method was used to calculate the 5-year incidence during follow-up after sexual initiation. In the first year of sexual activity, the incidence of cervical lesions was 24.1%. The incidence decreased to 3-8% over the following 4 years. The incidence density was 4.7 cases per 100 persons per year. The first abnormal cytological diagnosis showed atypical squamous cells of undetermined significance (ASCUS) in 5.5% (22) of patients, low-grade squamous intra-epithelial lesions (LSIL) in 28% (113), and high-grade squamous intraepithelial lesions (HSIL) in 3% (12). Eight (67%) cases of HSIL occurred during the first year of sexual activity. The incidence of cytopathological findings at beginning of sexual life is high, suggesting the importance of including sexually-active adolescents in cervical cancer prevention programs.