Introduction For patients with Stage III/IV Hodgkin lymphoma (HL), lack of response as assessed by PET scan following cycle 2 (PET2) of doxorubicin, bleomycin, vinblastine, and dacarbazine (ABVD) is predictive of poor clinical outcomes. SWOG Study S0816 utilized response-adapted therapy, where patients achieving a complete remission (CR) on PET2 (Deauville Score ≤ 3) continued 4 additional cycles of ABVD and those patients unable to achieve a complete remission (CR) on PET2 (Deauville Score > 3) were switched to therapy with escalated bleomycin, etoposide, doxorubicin, cyclophosphamide, vincristine, procarbazine, and prednisone (eBEACOPP) for 6 cycles. This study demonstrated a 2-year PFS of 64% in the PET2+ patients after switching to eBEACOPP, which was much higher than expected (Press 2016). Herein, we report the 5-year follow-up of survival and toxicities of the patients treated on S0816. Methods Patients with Stage III/IV HL were enrolled to S0816 from 7/1/09-12/2/12 (n=358). Eligible patients were age 18-60 years, had measurable disease, no prior lymphoma therapy, a Zubrod performance status of 0-2, and no serious medical comorbidities. Patients were treated with response-adapted therapy as above. A subset of 336 eligible and HIV-negative patients were analyzed. Of 336, 331 had central review of PET2. PFS and OS estimates were calculated using the Kaplan-Meier method. The two-sided Fisher's exact test was used to compare the rate of second cancer between patients switched to treatment eBEACOPP and those treated with continued ABVD after 2 initial cycles of ABVD. Results The median age of patients was 32 years (range, 18-60). All were Stage III/IV and 51% had an International Prognostic Score of ≥ 3. PET2 was negative in 271 patients (82%) and positive in 60 patients (18%). Of PET2+ patients, 49 (82%) switched to eBEACOPP as planned and 11 (18%) declined. Median follow-up is now 5.9 years (range, 0.2-8.3 years). Eighty-five (25%) patients have either progressed or died, for an estimated 5-year PFS of 74% (95% CI: 69%-79%; Figure 1). There were 64 (24%) events in those patients who were PET2- for a 5-year PFS of 76% (95% CI: 71-81%; Figure 2). There were 20 (33%) events in those patients who were PET2+ for a 5-year PFS of 65% (95% CI: 51-76%; Figure 2). One event occurred in a patient without central review of PET2. Nineteen patients have died for an estimated 5-year overall survival of 94% (95% CI: 91%-96%; Figure 1). Eleven (3%) patients died of HL including 6 (2%) that had PET2- and 5 (8%) that had PET2+. Other causes of death included: treatment-related toxicity [sepsis (eBEACOPP n=1), bleomycin lung injury (eBEACOPP n=1; ABVD n=1), and graft versus host disease (ABVD n=1)]; second primary cancers [(eBEACOPP n=2; cervical cancer and non-Hodgkin lymphoma (NHL)]; and unknown causes (ABVD n=2). Post-therapy grade 3 adverse events were uncommon but included 1 case each of heart failure, peripheral neuropathy, prolonged neutropenia, diarrhea, DVT (catheter-related) in patients who received continued ABVD and osteonecrosis of hips/shoulders in patients who received eBEACOPP. There were 13 (4%) cases of reported second cancers, including 7 (14%) in patients treated with eBEACOPP [1 each of myelodysplastic syndrome, kidney, melanoma, NHL, cervical, medullary thyroid, and basal cell carcinoma] and 6 (2%) in patients treated with ABVD (1 each of kidney, melanoma, NHL, bladder, prostate, and squamous cell carcinoma; 2-sided Fisher's exact P value = 0.001). Conclusions The long-term overall survival of the patients treated on S0816 remains high (94%) at 5 years. Despite historical data suggesting favorable clinical outcomes in patients with a negative PET2, nearly 25% of these patients experienced relapse events, demonstrating limitations of a PET-adapted approach and of standard frontline therapy with ABVD. In patients who were PET2+, PFS was favorable relative to historical series, but was associated with a high rate of secondary malignancies. Our results emphasize the importance of long-term follow-up in this disease, and the need for better biomarkers at diagnosis of HL and less toxic approaches for patients with a positive PET2. Acknowledgment In memory of Oliver W. Press, principal investigator of this trial, who died in 2017.