Background: SMA is a neurodegenerative disease caused by biallelic deletion/mutation of the survival motor neuron (SMN1) gene. In the phase 1 trial (NCT02122952), SMN GRT onasemnogene abeparvovec (AVXS-101) improved outcomes of 15 symptomatic SMA1 patients (3 at a lower dose [cohort 1] and 12 at the proposed therapeutic dose [cohort 2]). This report describes long-term follow-up study design and data from the phase 1 study. Methods: Patients in the phase 1 study could rollover into a long-term follow-up study (NCT03421977). The primary objective is to collect long-term safety data (serious adverse events, hospitalizations, and adverse events of special interest). Annual follow-up will occur for 15 years. Additionally, patient record transfers from local clinician(s) will be requested. Safety assessments include medical history and record review, physical examination, clinical laboratory evaluation, and pulmonary assessments. Efficacy assessments include physical examination to assess developmental milestones. Results: As of September 27, 2018, the oldest patients are 59.2 (cohort 1) and 52.1 (cohort 2) months old and free of permanent ventilation. Preliminary data, including survival and developmental milestones, will be presented. Conclusions: Patients treated with a one-time dose of AVXS-101 continue to gain strength, develop, and achieve new milestones, demonstrating a long-term, durable response.