In order to establish a sentinel physicians network in Castilla y León to collect systematically population-based morbidity data, a random sample of general practitioners (GP's) whose covered population was representative of the regional population was obtained. A cluster analysis with the ZBS (Zonas Básicas de Salud) was performed according to a list of variables considered important in the diseases incidence. Five clusters were obtained in the urban areas and twenty in the rural areas, where, after stratification, the GP's random sample was selected. The outcome of the distribution, within each cluster, between all Castilla y León GP's and the sentinel GP's did not show statistically significant differences. The statistically significant difference found between the age of all GP's and the 127 sentinel GP's, due to the voluntary participation, was not found in the comparison of the two populations covered by them, confirming the efficacy of this method in the selection of a representative population from a random sampling of population groups.