PURPOSE: To compare categorical severity classification systems for glaucoma. METHODS: This cross-sectional study included 1,921 eyes (49.5% right eye) from 1,137 participants from the Diagnostic Innovations in Glaucoma Study and African Descent and Glaucoma Evaluation Study. Standard automated perimetry fields were classified using the: (1) Advanced Glaucoma Intervention Study scoring system (AGIS), (2) Glaucoma Severity Staging system (GSS), and (3) Enhanced Glaucoma Severity Staging system (eGSS). Systems were characterized using the following continuous measures of severity: mean deviation, pattern standard deviation, and visual field index. Classifications between systems and with optic disc stereophotograph assessment were compared (κ) and some stages were consolidated to evaluate severity classification across systems (Wilcoxon test). RESULTS: Mean deviation, pattern standard deviation, and visual field index were significantly different between GSS and AGIS, and GSS and eGSS in normal and abnormal fields (P<0.005). Agreement between AGIS and eGSS was substantial (κ=0.715±0.012); agreement between GSS and eGSS (κ=0.559±0.014) and AGIS (κ=0.519±0.016) was moderate. eGSS tended to stage abnormal fields most severely followed by GSS and then AGIS (P<0.001). CONCLUSIONS: The presence of glaucomatous optic neuropathy increases with staging severity for all systems. However, different systems led to different severity staging. Of the systems examined in this study, eGSS may be the better choice for its ease of use for both clinicians and researchers. Copyright © 2012 by Lippincott Williams & Wilkins.