AbstractThe aim of this longitudinal case series was to describe long‐term functional outcome in a group of individuals with riboflavin transporter deficiency (RTD) treated with high‐dose oral riboflavin. Data were collected between 2012 to 2022. Eleven individuals with RTD were assessed at 12‐month intervals for monitoring of disease progression. Patients had commenced high‐dose oral riboflavin from the time of genetic diagnosis. Individuals for whom riboflavin supplementation was initiated early after disease onset had better outcomes compared to those in whom diagnosis was delayed. Despite ongoing riboflavin supplementation, the Charcot–Marie–Tooth disease Pediatric Scale (CMTPedS) total score and the subitems of balance and the 6‐Minute Walk Test distance as well as respiratory function worsened, while grip strength improved. There was evidence of improvement in hearing loss and optic atrophy limited to the first 12 months of treatment. While treatment with riboflavin slowed disease progression, patients were left with residual disability. To track disease progression and response to riboflavin supplementation over time, we recommend using the RTD Pediatric Scale and provide a list of clinical measures for regular surveillance of children with RTD.