OBJECTIVE Liquid-based cytologic methods are increasingly used, and classification of squamous intraepithelial lesions (SIL) affects patient management. This study compared interobserver reproducibility in SIL subclassification on conventional (CV) and ThinPrep (TP) cytologic specimens. METHODS Four reviewers independently subclassified SIL on 69 CV and 60 TP Paps. Specimens were retrieved by computer search of biopsy-confirmed SIL cases. A consensus interpretation of low-grade SIL (LSIL) or high-grade SIL (HSIL) was assigned when three or four observers agreed. RESULTS All four observers agreed in 40 of 69 CV with consensus reached in 56 of 69 CV Paps (81%; 20 LSIL, 36 HSIL). For TP Paps, 38 of 60 had 100% agreement, with consensus reached in 56 of 60 TP Paps (93%; 28 LSIL, 26 HSIL, 2 SIL, difficult to grade). kappa values for all four observers were 0.48 for CV (fair agreement) and 0.63 for TP (substantial). Pairwise kappa values ranged from 0.44 to 0.60 for CV and 0.54 to 0.76 for TP. Most of the nonconsensus cases included SIL, difficult to grade interpretations; in several, the original cytologic or biopsy SIL classification, or both, was also indeterminate, or cytologic and biopsy results did not correlate exactly. High-grade biopsies followed 15% of LSIL CV and 36% of LSIL TP. CONCLUSIONS Interobserver reproducibility in SIL subclassification may be better on TP Paps; however, both CV and TP have indeterminate lesions with low interobserver agreement. The TP specimens did not show improved correlation with histologic analysis, and specimens with consensus do not always have correlating biopsy findings.