Socioeconomic level is a variable related with health. In order to obtain an indicator of this variable which is suitable for epidemiological and public health studies, we propose to assign the "social class" on the basis of occupation. Having as a reference the British Classification of Social Class we have used the National Classification of Occupations (CNO) for social classification. Once elaborated, we have used the classification in two population-based studies. More than 96% of individuals studied could be classified. We evaluate the usefulness of this indicator and its limitations. Also, we have indicate the need of validating the proposed classification in the future.