Spinal muscular atrophy is a neurodegenerative disease resulting from irreversible loss of anterior horn cells owing to biallelic deletions/mutations in the survival motor neuron (SMN) 1 gene. Gene replacement therapy using an adeno-associated virus vector containing the SMN gene was approved by the US Food and Drug Administration in May 2019. We report 2 cases of transient, drug-induced liver failure after this therapy.