This article offers an approach through administrative and cultural history to the problems associated with gathering and processing data for the Spanish national census of 1920, and by implication for earlier Spanish censuses. It focuses on the Basque province of Guipúzcoa, making use of correspondence between the central statistical office in Madrid, the provincial jefe de estadística and the localities, and of reports on three problematic towns within the province. The issues that emerge regarding ‘undercounting’, the definition of administrative boundaries and the classification of demographic characteristics are set in the wider context of census-taking practices and problems elsewhere in Spain and in other cultures.