Background <p>Spinal muscular atrophy (SMA) is a rare genetic disease resulting in loss of motor function and, in severe cases (e.g., SMA type 1), infantile death. While treatments like nusinersen and onasemnogene abeparvovec improve prognosis for patients with SMA, costs for these medications can contribute to economic burden.</p> Objective <p>Direct costs were compared for onasemnogene abeparvovec, a one-time gene replacement therapy, versus nusinersen, a lifelong therapy, for patients with SMA type 1 and/or three or more <i>survival motor neuron 2</i> (<i>SMN2</i>) gene copies in the Netherlands.</p> Methods <p>A cost comparison analysis model of 1-year incident patient population from the Netherlands was used to compare costs of onasemnogene abeparvovec versus nusinersen for patients eligible for onasemnogene abeparvovec immediately after diagnosis. Multiple analyses were conducted for economic outcomes (e.g., base-case, break-even, deterministic sensitivity, probabilistic sensitivity, scenario analyses).</p> Results <p>Cost differences of –€2.9&#xa0;million (undiscounted) and –€1.5&#xa0;million (discounted) per patient with SMA type 1 treated with onasemnogene abeparvovec versus nusinersen over a 20-year time horizon were identified (base-case). Reduced costs with onasemnogene abeparvovec versus nusinersen were evident after 8.25 years.</p> Conclusion <p>Onasemnogene abeparvovec was less costly than nusinersen after 8.25 years of treatment of patients with SMA type 1 in the Netherlands.</p>