Introduction - Social disadvantage indicators enable routine monitoring of health inequalities. This study defines an indicator and assesses its association with mortality by cause of death across mainland France. Methods - The FDep99 social disadvantage indicator is defined at the municipal level as the first component of a principal component analysis of four variables. The spatial association between social disadvantage and mortality is examined by urban unit category, region, and cause of death, over the 1997–2001 period. Results - FDep99 accounts for 68% of the total inertia of the scatter plot. Its construction is insensitive to the urban unit category considered. It is positively and quasi-linearly associated with mortality levels, with the SMR (standardized mortality ratio) being 24% higher for municipalities in the most disadvantaged quintile compared to those in the least disadvantaged quintile. The association is similar within each urban unit category and each region. It is positive for nearly all causes of death and stronger for men. Conclusion - The method used to construct the proposed index allows for a summary of the socioeconomic heterogeneity of the territory without any a priori link to mortality. The index enables routine monitoring of social inequalities in health; it is currently the subject of further investigations. (R.A.)