The fiscal reform of 1683 originated a documentation of great demographic value that has not yet been published. This paper presents and evaluates the new source in order to know the evolution of Castilian population in a period without credible data until now. The Vecindario of 1683 is verified twice in the study, firstly by the birth rate and secondly by comparing the population growth with that of baptisms. The result demonstrated that the decline of 1591'1631 had not yet been recovered in 1683 and that the subsequent growth was faster in 1683'1752 than in 1752'1787.