Background and aimsIn ADHERE, subcutaneous efgartigimod PH20 (1000 mg once weekly) was effective and well tolerated in participants with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). ADHERE+ is an open-label extension of ADHERE assessing long-term safety and efficacy.MethodsEligible participants from ADHERE run-in period (prior CIDP treatments discontinued), stage A (open-label efgartigimod), and stage B (stage A responders randomized to placebo or efgartigimod) could roll over to ADHERE+ and receive efgartigimod. The primary outcome was to assess long-term safety and tolerability. Efficacy outcomes evaluated adjusted Inflammatory Neuropathy Cause and Treatment (aINCAT) score, Inflammatory Rasch-built Overall Disability Scale (I-RODS) score, and mean grip strength (GS) from ADHERE run-in baseline through ADHERE+ week 36.ResultsAt interim data cut-off (February 16, 2024), 228/229 eligible participants advanced to ADHERE+. Prolonged efgartigimod exposure did not increase the incidence (n = 171/228 [75.0%]) or severity of TEAEs (grade ≥ 3 TEAEs, n = 41/228 [18.0%]); regardless of immunoglobulin G level, there was a low incidence of infections. Stage A responders reported long-term clinically meaningful improvements in mean aINCAT (decreased by 1.2 points), I-RODS centile metric (increase of 8.8 points), and GS scores (increase of 17.5 kPa) from run-in baseline to ADHERE+ week 36, irrespective of stage B treatment.InterpretationInterim results from ADHERE+ indicate long-term efgartigimod PH20 treatment in participants with CIDP was well tolerated (maximum exposure = 187.3 weeks; mean (SD) treatment duration = 58.0 [33.8] weeks). Clinically meaningful improvements in disability and strength were observed across assessments, regardless of stage B treatment or prior treatment status, with greater improvements seen over time.