At our institution, 17% of cervical conization specimens are reported as negative for dysplasia or malignancy. To identify sources of error, we reviewed 53 negative conization specimens and their prior and follow-up cytology, biopsy, and endocervical curettage specimens. Examination of deeper-level sections and p16 immunostaining were performed on all conization specimens and selected biopsy specimens. Dysplasia was detected in 26% (14/53) of conization specimens. Twenty-eight percent (15/53) of cones were truly negative, and the presurgical material had been overcalled as high-grade squamous intraepithelial lesions (HSIL). Forty-five percent (24/53) of cones were truly negative and HSIL was confirmed in the presurgical material. Of these, 11% (6/53) showed subsequent evidence of residual dysplasia and 26% (14/53) were negative on further follow-up. Deeper-level sections, p16 immunostains, and consensus review may help identify squamous dysplasia in conization specimens and may prevent the overdiagnosis of HSIL on cervical biopsies.