ACOX1 (acyl-CoA oxidase 1) encodes the first and rate-limiting enzyme of the very-long-chain fatty acid (VLCFA) β-oxidation pathway in peroxisomes and leads to H2O2 production. Unexpectedly, Drosophila (d) ACOX1 is mostly expressed and required in glia, and loss of ACOX1 leads to developmental delay, pupal death, reduced lifespan, impaired synaptic transmission, and glial and axonal loss. Patients who carry a previously unidentified, de novo, dominant variant in ACOX1 (p.N237S) also exhibit glial loss. However, this mutation causes increased levels of ACOX1 protein and function resulting in elevated levels of reactive oxygen species in glia in flies and murine Schwann cells. ACOX1 (p.N237S) patients exhibit a severe loss of Schwann cells and neurons. However, treatment of flies and primary Schwann cells with an antioxidant suppressed the p.N237S-induced neurodegeneration. In summary, both loss and gain of ACOX1 lead to glial and neuronal loss, but different mechanisms are at play and require different treatments. Display omitted •Loss of dACOX1 leads to glial degeneration via accumulation of VLCFA•A de novo ACOX1N237S gain-of-function mutation also leads to glial loss•Glial loss in ACOX1N237S is associated with elevated ROS, not an increase in VLCFA•N-acetyl cysteine amide, an antioxidant, suppresses glial defects in ACOX1N237S flies Chung et al. show that ACOX1, a peroxisomal protein required for degrading VLCFA, is mostly expressed in glial cells. Its loss causes an accumulation of VLCFA and glial loss. An ACOX1 gain-of-function mutation identified in three individuals leads to elevated ROS and glial loss and is potently suppressed by antioxidants.