Context .- Misinterpretation of high-grade squamous intraepithelial lesion (HSIL) is an important problem in daily practice and in the College of American Pathologists (CAP) PAP Proficiency Test (PAP-PT). Objective .- To investigate factors related to misinterpretation of HSIL through responses in a proficiency test versus an educational environment. Design .- We retrospectively evaluated 28 000 responses in the PAP Education Program (PAP-Edu) and 59 140 responses in PAP-PT from 1147 field-validated HSIL slides from 2007 to 2014. The related factors, such as program types, preparation types, participant types, and program years, were evaluated. Results .- Overall, 4.0% (2379 of 59 140) of responses for HSIL slides from PAP-PT were misinterpreted as either low-grade squamous intraepithelial lesion (LSIL) or negative, significantly more than those from PAP-Edu (3.2%; 898 of 28 000). However, the false-negative rate (misinterpreted as negative) was 0.9% (519 of 59 140) for PAP-PT, lower than that for PAP-Edu (1.0%; 266 of 28 000). The misinterpretation rates in PAP-PT trended down with time. Misinterpretation rates did not vary significantly by preparation methods. The misinterpretation rate for HSIL in the pathologists' responses was lower than that in cytotechnologists' in PAP-PT. More HSIL was misinterpreted as LSIL than as benign in both programs. Cytotechnologists interpreted HSIL as LSIL twice as much as pathologists. The most common false-negative misinterpretations were negative for intraepithelial lesion or malignancy and reparative change. Conclusions .- The higher LSIL misinterpretation rate by cytotechnologists may be related to the differences in reporting responsibilities and proficiency test grading criteria. The trend of gradually decreasing misinterpretation rates of a reference diagnosis of HSIL in the PAP-PT program may be related to higher test-taking confidence and better preparation through educational programs. The fact that pathologists performed better than cytotechnologists in PAP-PT, but not in PAP-Edu, may reflect a heightened approach and attentiveness in the test-taking environment for pathologists.