Background: To search for and quantify the importance of socio-economic inequality in seat belt use in young Belgians (15–24 years old). Methods: Using the data of the National Health Survey of Belgium (1997), socio-economic indicators were constructed combining characteristics of the individual and the household to which s/he ‘belongs’ (equivalent household income, employment, poverty, subjective poverty, and socio-economic insecurity) and the corresponding concentration curves and indices were calculated. Results: Significant differences were found in seat belt use related to young people's socio-economic status. The failure to buckle up was concentrated most heavily in the least favoured socio-economic groups. Such inequality was a constant finding, although its magnitude varied according to the socio-economic indicator used, going from –0.142 for the variable ‘occupational category of the household’ to –0.028 for the variable ‘poverty’. Conclusions: This finding of true socio-economic inequality in the use of seat belts by 15–24 year olds should prompt special vigilance in health promotion approaches aimed at changing behaviour. It also underlines the importance of being attentive to the socio-economic indicators that are chosen to quantify such inequality in intra- and inter-country comparisons.