Objective: To evaluate a single item instrument, the Global Activity Limitation Indicator (GALI), to measure long-standing health related activity limitations, against several health indicators: a composite morbidity indicator, instruments measuring mental health (SCL-90R, GHQ-12), physical co-morbidity and physical limitations (ADLs, SF-36). Methods: Cross-sectional data (n = 9168) of the 2001-National Health Interview Survey in Belgium was used to compare the GALI with other health indicators across gender, age, educational attainment and language. Results: Responses to the GALI were similar to responses to other indicators of physical limitations (Limitations in Activities of Daily Living (by severity or by number of limitations), the SF-36 physical domain), to an indicator of chronic physical comorbidity and to indicators of mental health. The probability of reporting absence of long-standing activity limitation with the GALI was high in subjects without physical limitations or physical or mental conditions. This probability decreased as the severity or number of limitations, the number of physical or mental conditions increased. Conclusions: The GALI performs appropriately against other health indicators and appears to refl ect long-standing activity limitation associated with both mental and physical conditions. © Birkhäuser Verlag, Basel 2006.