One explanation for a lack of consistency in SES-health associations in youth is that parent-based income and occupation measures are inadequate. The Family Affluence Scale (FAS), a four-item measure of family wealth, has been developed in the WHO Health Behaviour in School-aged Children Study as an alternative measure. The aim of this paper is to examine the criterion and construct validity of the FAS as a measure of national absolute wealth in 35 countries. A general measure of national wealth, Gross Domestic Product (GDP) for the year 2001, corrected for Purchasing Power Parity, was used as a gold standard. For the composite FAS index, there was a similar strength in country rank order correlation (0.87) as with GDP, and a Kappa agreement coefficient of 0.57, indicating good criterion validity. FAS index associations with national health indicators were systematically higher than the values obtained with GDP. These features suggest that FAS can be used with confidence in aggregate analyses of HBSC data that focus on relationships between SES and adolescent health.