Our objective was to describe the evolution of cervical SIL within 24 months of the initial diagnosis, in a cohort study of 147 sexually active adolescents attending a public health service in Rio de Janeiro, between 1993 and 2006. The participants were divided in two groups, according to whether cervical biopsy was performed or not. The median of the interval between sexual debut and the atypical cytopathology was 12 months and in 8.2% of patients there was a diagnosis of HSIL at the first abnormal smear. After a two-year follow-up by cytology, the regression (ASCUS 91%, LSIL 63.6%, HSIL 50%) and progression (LSIL 6.1%) were verified. In the group undergoing biopsy, the final histological regression reached 59.4% for CIN1 and 71.4% for CIN2, while the progression from CIN1 to CIN 2/3 was 3.1%. Our results corroborate the recommendation for conservative management in compliant adolescents due to a high regression rate. However, there should be maintained a careful follow-up based on the possible evolution of the lesion.