Background The purpose of this study was to assess the construct validity of the Aberrant Behaviour Checklist‐utility index (ABC‐UI) by examining the relationship between healthcare resource utilisation by patients with fragile X syndrome (FXS) as well as burden experienced by their caregivers. Method In 2011, a total of 350 US caregivers of individuals with FXS completed a questionnaire that captured information on FXS‐related burden as well as the ABC‐Community. Using the ABC‐UI, a condition‐specific outcome measure derived from the ABC‐Community, five utility index categories were created: very low (0.00–0.33); low (0.34–0.66); moderate (0.67–0.77); high (0.78–0.89); and very high (0.90–1.00). Multivariable regression models examined the association between the utility value and nine burden‐related outcomes. Results Approximately 2% of individuals with FXS were in the very low utility index category, 31% low, 27% moderate, 38% high and 3% very high. The median utility value was 0.74. Women with FXS and adults 18 years and older had higher values. Regression results indicate that individuals with higher utility values were more likely to have fewer specialist visits, use fewer prescription medications, need fewer hours of unpaid caregiving, inflict fewer caregiver injuries and have caregivers with fewer mental health provider visits. Conclusions The ABC‐UI appears to function well as condition‐specific outcome measure, and as an indicator of health‐related quality‐of‐life and economic burden in individuals with FXS. Among patients with FXS in the US and their caregivers, significant differences in health care resource utilisation and burden exist across health state utility categories.