Spinal muscular atrophy (SMA) is the most common genetic motoneuron degenerative disorder, but the mechanism(s) of motoneuron degeneration is unclear. We previously generated SMA model mice, which genotypically and phenotypically mimicked human SMA patients, by a combination of knockout and transgenic techniques. Here, we used these SMA model mice to decipher the apoptotic mechanism(s) involved in SMA motoneuron degeneration. We found a significant increase in proapoptotic Bax expression in the spinal cords of SMA mice in comparison with their wild-type littermates. After crossing SMA mice with Bax knockout mice, we produced in vivo evidence indicating that Bax protein plays an important role in the degeneration of SMA spinal motoneurons. Progeny Bax-deficient SMA mice showed milder disease severity, longer life spans, and significant increases in spinal motoneuron densities compared to SMA littermates with wild-type Bax genes. Our results strongly suggest that suppression of Bax-involved apoptosis has the potential for amelioration of SMA.