Purpose To determine whether treatment of anal high-grade squamous intraepithelial lesions (HSIL), vs active monitoring, is effective in reducing incidence of anal cancer in persons living with HIV, the US National Cancer Institute funded the Phase III ANal Cancer/HSIL Outcomes Research (ANCHOR) clinical trial. As no established patient-reported outcomes (PRO) tool exists for persons with anal HSIL, we sought to estimate the construct validity and responsiveness of the ANCHOR Health-Related Symptom Index (A-HRSI). Methods The construct validity phase enrolled ANCHOR participants who were within two weeks of randomization to complete A-HRSI and legacy PRO questionnaires at a single time point. The responsiveness phase enrolled a separate cohort of ANCHOR participants who were not yet randomized to complete A-HRSI at three time points: prior to randomization (T1), 14–70 (T2), and 71–112 (T3) days following randomization. Results Confirmatory factor analysis techniques established a three-factor model (i.e., physical symptoms, impact on physical functioning, impact on psychological functioning), with moderate evidence of convergent validity and strong evidence of discriminant validity in the construct validity phase ( n  = 303). We observed a significant moderate effect for changes in A-HRSI impact on physical functioning (standardized response mean = 0.52) and psychological symptoms (standardized response mean = 0.60) from T2 ( n  = 86) to T3 ( n  = 92), providing evidence of responsiveness. Conclusion A-HRSI is a brief PRO index that captures health-related symptoms and impacts related to anal HSIL. This instrument may have broad applicability in other contexts assessing individuals with anal HSIL, which may ultimately help improve clinical care and assist providers and patients with medical decision-making.