Background Assessment of the effectiveness of chemotherapy for advanced stage Hodgkin lymphoma may be obscured by the incorporation of radiotherapy in primary treatment when standard endpoints such as progression free survival (PFS) are used. Ineffective chemotherapy may be “rescued” by the addition of radiotherapy residual PET-positive masses. Use of a modified PFS endpoint (mPFS), in which addition of radiotherapy after the conclusion of chemotherapy is scored as an event, more accurately assesses chemotherapy effectiveness. Methods We examined the records of 207 consecutive adult patients (age 16-82 y, median 36; males 55%) with advanced stage HL (IIIA, 23%; IIIB, 24%; IVA, 17%; IVB, 36%) treated between 2004 and 2015 with ABVD with or without involved field radiotherapy (IFRT). Standard PFS events were the first of relapse or progression (REL/PROG) despite primary treatment (ABVD +/- IFRT) or death from any cause; mPFS events were the first of radiotherapy given after ABVD if the post-ABVD PET was positive (PET-RT), relapse or progression after primary treatment (REL/PROG) or death from any cause. Results We observed 56 PFS events (REL/PROG, 54; death 2) and 64 mPFS events (PET-RT, 17; REL/PROG 45; death 2). Thus, 8/64 (13%) of the relevant events (failure of the primary chemotherapy to cure the lymphoma) were unappreciated using the standard PFS endpoint. This resulted in an overestimate of the effectiveness of primary chemotherapy (2-y PFS 78%; 2-y mPFS 72%) and of the duration of survival free of death, relapse or progression (time to 25% treatment failures using standard PFS = 2.8 y; using mPFS = 1.1 y) (Figure 1). These overestimates occur because the addition of radiotherapy to the primary chemotherapy “rescues” patients whose primary chemotherapy has failed (PET positive residual disease still present after completion of the primary chemotherapy) and creates the appearance of successful treatment. Conclusion Use of the mPFS endpoint provides a superior assessment compared to standard PFS if the goal of a clinical trial testing treatment for advanced stage Hodgkin lymphoma is to identify the most effective chemotherapy regimen. This is particularly important in an era when post-chemotherapy evaluation with PET has become standard and major efforts are being made to determine the potential benefit of adding novel agents such as antibody-drug conjugates and checkpoint inhibitors to standard chemotherapy.