The new Bethesda System (BS) terminology has opened a series of problems about the Abnormal/Atypical Squamous Cells of Undetermined Significance (ASCUS) and Low-Grade Squamous Intraepithelial Lesion (L-SIL) categories, particularly on their treatment and follow-up. Moreover in these field a non negligible portion of lesions progress to High-Grade Lesion (H-SIL). With the aim of comparing the data, we examined samples observed in our Ambulatories with 6-12 months follow-up. We observed retrospectively 11.197 Pap test from January 1995 to June 1997, mostly first visits. All received a Pap test, colposcopy and gynaecological examination. Biopsy and histological standard examinations were performed when necessary. Alterations classified as mild epithelial abnormalities, of both ASCUS and L-SIL categories, were observed in 146 cases: 78 ASCUS and 68 L-SIL. Of these 48 were CIN 1/mild dysplasia (25 HPV associated) and 20 were HPV lesions, according to BS. All the cases with persistent ASCUS and L-SIL underwent a second control. In these 45 cases we observed 2 new cases of H-SIL and one of L-SIL; so 82.3% and 30.7% of ASCUS and L-SIL regressed to negatives. In the stable group 11.8% and 60.4% were ASCUS and L-SIL. Moreover 5.9% of ASCUS and 4.8% of L-SIL progressed to H-SIL. No cases of invasive cancer were observed. Our data show that ASCUS and L-SIL diagnosis can identify 6% and 4.8% of H-SIL respectively. In addition 17-18% of ASCUS were stable or progressed. These two categories, as already demonstrated in other studies, are mostly at risk.