Background: Although the cytological finding of atypical squamous cells of undetermined significance (ASCUS) occurs in around 5% of women undergoing cytological screening, the clinical evolution is unknown. The objective of this study was to evaluate women with a diagnosis of ASCUS and compare the histological findings and clinical evolution over six and 12 months. Methods: 1,244 patients with ASCUS (two diagnostic cytologists) were evaluated in this study with Pap Smears and colposcopy (biopsy if necessary) at the first visit, and thereafter at second and third visits (6 and 12 months after the first visit). Comparisons were made between cytological findings and histological diagnoses at every visit during the study and during evolution. Results: At the first visit, 60.3% of the biopsied patients presented histological findings of CIN I, 17.46% CIN II/III and 6.3% invasive neoplasia. At the second visit, 48.3% of the biopsied patients presented CIN I, 24.1 % CIN II/III and none invasive neoplasia. At the third visit, 68.9% of the biopsied patients presented CIN I, 13.8% CIN II/III and none invasive carcinoma. If we consider all visits, a total of 213 col­poscopy-guided biopsies were performed, representing 17.1 % of all the patients included. Of these, 164 (13.2%) presented a histological diagnosis of cervical intraepithelial neoplasia (CIN) and eight (0.6%) presented with invasive carcinoma. Conclusion: We conclude that CIN or invasive lesions frequently occur in women with a diagnosis of ASCUS. Immediate review of the thin sec­tions, new cytological tests or colposcopy and rigorous follow-up should be considered when making a diagnosis of ASCUS.