Background: Efgartigimod, a human immunoglobulin (Ig)G1 antibody Fc fragment, blocks the neonatal Fc receptor, reducing IgGs involved in chronic inflammatory demyelinating polyneuropathy (CIDP). The multi-stage, double-blinded, placebo-controlled ADHERE (NCT04281472) and open-label extension ADHERE+ (NCT04280718) trials (interim analysis cutoff: February 16, 2024) assessed efgartigimod PH20 SC in participants with CIDP. Methods: Participants with active CIDP received open-label, weekly efgartigimod PH20 SC 1000 mg during ≤12-week run-in (stage-A). Responders were randomized (1:1) to efgartigimod or placebo for ≤48 weeks (stage-B). Participants with clinical deterioration in stage-B or who completed ADHERE entered ADHERE+. Week 36 changes from run-in baseline (CFB) in adjusted Inflammatory Neuropathy Cause and Treatment (aINCAT), Inflammatory Rasch-built Overall Disability Scale (I-RODS), and grip strength scores were evaluated. Results: Of 322 stage-A participants, 221 were randomized and treated in stage-B, and 99% entered ADHERE+. Mean CFB (SE) in aINCAT, I-RODS, and grip strength scores were -1.2 (0.15) and 8.8 (1.46) and 17.5 (2.02), respectively, at ADHERE+ Week 36 (N=150). Half the participants with clinical deterioration during ADHERE stage-B restabilized on efgartigimod from ADHERE+ Week 4. Conclusions: Interim results from ADHERE+ indicate long-term effectiveness of efgartigimod PH20 SC in clinical outcomes in participants with CIDP.