In the absence of individual data, ecological or contextual measures of socioeconomic level are frequently used to describe social inequalities in health. This work focuses on the methodological aspects of the development and validation of a French small-area index of socioeconomic deprivation and its application to the evaluation of the socioeconomic differentials in health outcomes. This index was derived from a principal component analysis of 1999 national census data from the Strasbourg metropolitan area in eastern France, at the census block level. Composed of 19 variables that reflect the multiple aspects of socioeconomic status (income, employment, housing, family and household, and educational level), it can discriminate disadvantaged urban centres from more privileged rural and suburban areas. Several statistical tests (Cronbach's alpha coefficient, convergent validity tests with other deprivation indices from the literature) provided internal and external validation. Its successful application to another French metropolitan area (Lille, in northern France) confirmed its transposability. Finally, its capacity to capture the social inequalities in health when applied to myocardial infarction data shows its potential value. This study thus provides a new tool in French public health research for characterising neighbourhood deprivation and detecting socioeconomic disparities in the distribution of health outcomes at the small-area level.