[Show abstract][Hide abstract] ABSTRACT: Brown-Vialetto-Van Laere syndrome (BVVLS) is a genetic condition caused by a mutation in the C20orf54 gene, which also codes for an intestinal riboflavin transporter. We report the case of a female who presented at 22 months with acute-onset stridor and generalized muscle weakness, in whom a genetic diagnosis of BVVLS was made, and whose symptoms improved on therapy with high-dose riboflavin. She had previously been developing normally and was able to walk at 11 months, then developed progressive muscle weakness at 22 months, and within 2 weeks was unable to sit without support. She also demonstrated stridor and paradoxical breathing indicating diaphragmatic weakness, and required continuous non-invasive ventilation (NIV) through a tracheostomy. After treatment with riboflavin she was able to walk unaided, and her Gross Motor Functional Classification level improved from level IV to level I, having fully regained the motor function she showed before symptom onset. There were no longer signs of diaphragmatic paralysis while on NIV, and she was able to tolerate 10-minute periods off NIV before paradoxical breathing again became apparent. We therefore recommend that in all cases suspected to be in the BVVLS or Fazio-Londe spectrum, early treatment with high-dose riboflavin must be considered.