BACKGROUND AND OBJECTIVES: Historically, autosomal recessive 5q-linked spinal muscular atrophy (SMA) has been the leading inherited cause of infant death. SMA is caused by the absence of the SMN1 gene, and SMN1 gene replacement therapy, onasemnogene abeparvovec-xioi, was Food and Drug Administration approved in May 2019. Approval included all children with SMA age ,2 years without end-stage weakness. However, gene transfer with onasemnogene abeparvovec-xioi has been only studied in children age #8 months. METHODS:In this article, we report key safety and early outcome data from the first 21 children (age 1-23 months) treated in the state of Ohio.RESULTS: In children #6 months, gene transfer was well tolerated. In this young group, serum transaminase (aspartate aminotransferase and alanine aminotransferase) elevations were modest and not associated with g glutamyl transpeptidase elevations. Initial prednisolone administration matched that given in the clinical trials. In older children, elevations in aspartate aminotransferase, alanine aminotransferase and g glutamyl transpeptidase were more common and required a higher dose of prednisolone, but all were without clinical symptoms. Nineteen of 21 (90%) children experienced an asymptomatic drop in platelets in the first week after treatment that recovered without intervention. Of the 19 children with repeated outcome assessments, 11% (n = 2) experienced stabilization and 89% (n = 17) experienced improvement in motor function. CONCLUSIONS:In this population, with thorough screening and careful post-gene transfer management, replacement therapy with onasemnogene abeparvovec-xioi is safe and shows promise for early efficacy.