The histologic and/or cytologic follow-up of 127 cases of cervical lesions termed "squamous intraepithelial lesion of indeterminate grade" (SIL) on Papanicolaou (Pap) smears by the 2001 Bethesda System was compared with 150 control cases of low-grade SIL (LSIL), high-grade SIL (HSIL), and atypical squamous cells, cannot exclude HSIL (ASC-H). A follow-up diagnosis of cervical intraepithelial neoplasia (CIN) 2 or higher was identified in 22.8% of SIL cases, which was 2.6 times higher than LSIL, 3 times lower than HSIL, and 1.5 times lower than ASC-H. A follow-up diagnosis of CIN 1 was identified in 31.5% of SIL cases, which was 2 times lower than the LSIL group, 1.5 times higher than the ASC-H cases, and 1.8 times higher than the HSIL group. We found that 22.0% of cases diagnosed as SIL were followed up by Pap smears rather than colposcopy and biopsy, compared with about 1% of LSIL and HSIL cases. Because SIL cases have a significant risk of harboring CIN 2 or greater, we recommend follow-up by colposcopy and biopsy.