OBJECTIVE: We sought to estimate the incidence of cervical intraepithelial neoplasia (CIN) and treatment outcomes in adolescents with abnormal cytology. METHODS: Adolescent women (ages 14-21 years) referred to colposcopy clinic for abnormal cytology from 1992 to 2004 were identified by computerized database. Only adolescents with biopsy-proven CIN were evaluated. Demographic and risk factor data were obtained from medical records. Referral cytology, histology on biopsy and loop electrosurgical excisional procedure (LEEP), and follow-up cytology were analyzed and compared. Statistical analysis was performed by chi(2) or Fisher exact test, Student t tests, and logistic regression. RESULTS: Of 1,678 adolescents, 517 had biopsy-proven CIN and follow-up. Seventy-seven patients were referred with atypical squamous cells of undetermined significance (ASCUS) cytology; 174 patients were referred with low-grade squamous intraepithelial lesions (LSIL), 258 with high-grade squamous intraepithelial lesions (HSIL) and eight with atypical glandular cells (AGC). The rate of CIN 2/3 in patients with ASCUS, LSIL, and HSIL was 35% (95% confidence interval 24-46%), 36% (29-43%), and 50% (44-56%), respectively. A total of 192 patients with biopsy-proven CIN 2/3 underwent a LEEP. No patients were diagnosed with cervical carcinoma. Fifty-five percent (95% confidence interval 48-62%) of patients had abnormal cytology on follow-up, suggesting recurrence or reinfection. CONCLUSION: Adolescents with abnormal cytology have a high incidence of CIN2/3 and high rates of abnormal cytology after LEEP. Cervical intraepithelial neoplasia 2/3 is common in adolescents with abnormal cytology, yet no cases of cancer were identified. Importantly, LEEP fails to meet its therapeutic goals given a high incidence of abnormal follow-up cytology and may represent overly aggressive therapy because the majority of human papillomavirus infections are transient with high regression rates. LEVEL OF EVIDENCE: III.