BackgroundClassical galactosemia (CG), an inborn error of galactose metabolism, results in long-term complications including cognitive impairment and movement disorders, despite early diagnosis and dietary treatment. Two decades ago, lower motor-, cognitive- and social health related quality of life (HRQoL) was demonstrated in pediatric and adult patients. Since then, the diet has been relaxed, newborn screening was implemented and new international guidelines resulted in major changes in follow-up. The aim of this study was to assess HRQoL of CG by means of online self- and/or proxy-HRQoL-questionnaires focusing on the main areas of concern of CG (i.e. anxiety, depression, cognition, fatigue, social- and upper extremity function) within the patient-reported outcomes measurement information system (PROMIS®) and generic HRQoL-questionnaires (TAPQOL, TACQOL, TAAQOL).ResultsData of 61 Dutch patients (aged 1–52 years) were collected and compared to available Dutch or US reference populations. On the PROMIS-questionnaires, children reported more fatigue (P= 0.044), lower function in upper extremities (P= 0.021), more cognitive difficulties (P= 0.055,d= 0.56) and higher anxiety (P= 0.063,d= 0.52) than reference children although the latter findings remained non-significant. Parents of CG patients reported lower quality of peer relationships of their children (P< 0.001). Both children and parents reported lower cognitive functioning (P= 0.005,P= 0.010) on the TACQOL. Adults reported on PROMIS domains lower cognitive functioning (P= 0.030), higher anxiety (P= 0.004) and more fatigue (P= 0.026). Cognitive difficulties were reported on the TAAQOL by adults (P< 0.001), as well as physical-, sleeping and social difficulties.ConclusionsCG remains to impact the HRQoL of pediatric and adult patients negatively on several domains including cognition, anxiety, motor function and fatigue. A lower social health was mainly reported by parents, and not by patients themselves. The Covid-19 pandemic might have amplified the results on anxiety although higher levels of anxiety fit pre-pandemic findings. The reported fatigue is a new finding in CG. Because the effect of lockdown fatigue could not be eliminated and fatigue is a frequent finding in patients with chronic disorders, future studies are warranted. Clinicians and researchers should be attentive to both pediatric and adult patients, and the age-dependent difficulties they might encounter.Supplementary InformationThe online version contains supplementary material available at 10.1186/s13023-023-02749-8.