Background: High risk for relapse is observed in cHL patients (pts) with SER to initial chemotherapy and organ toxicities may be higher following dose intensification. Methods: The phase 2 KEYNOTE-667 study will enroll 440 pts aged 3 to 17 (children) or 18 to 25 years (young adults) with newly diagnosed, confirmed stage IA, IB, or IIA cHL without bulky disease (Group 1 [low-risk]) or stage IIEB, IIIEA, IIIEB, IIIB, IVA, or IVB cHL (Group 2 [high-risk]); measurable disease; and performance status per Lansky Play-Performance Scale ≥50 (age ≤16 years) or Karnofsky score ≥50 (age >16 years). Pts will receive induction with doxorubicin, bleomycin, vinblastine, dacarbazine (ABVD; Group 1) or vincristine, etoposide/etoposide phosphate, prednisone/prednisolone, doxorubicin (OEPA; Group 2) for 2 cycles, then early response assessment by PET/CT/MRI. Pts with rapid early response (Deauville score 1-3) will receive standard therapy. Pts with SER (Deauville score 4-5) will receive consolidation with pembro 2 mg/kg Q3W up to 200 mg (children) or 200 mg Q3W (young adults) plus 2 cycles AVD (Group 1) or 4 cycles cyclophosphamide, vincristine, prednisone/prednisolone, dacarbazine (COPDAC-28; Group 2). PET/CT for late response assessment (LRA) will be performed after consolidation. After LRA, Group 1 pts with SER and Group 2 pts with Deauville score 4-5 will receive radiotherapy (RT). All pts will receive maintenance with pembro Q3W concomitantly with RT. Pembro will continue up to 17 administrations, with an option to stop after 24 weeks due to CR, or until progression, unacceptable toxicity, or withdrawal. The primary endpoint is ORR per Cheson 2007 IWG criteria by group in SER pts. Secondary endpoints are SERs with PET negativity after consolidation, 2-yr event-free survival (EFS), OS, and RT frequency and details by group, RERs with PET negativity after ABVD induction, 3-yr EFS by investigator, and OS by risk group, and serum TARC levels at screening in SERs by risk group. ORR with 95% CI will be estimated by Clopper-Pearson method. EFS and OS will be estimated by Kaplan-Meier method. Safety will be assessed in all treated pts. Clinical trial information: NCT03407144