Luis F. Lindley Cintra established a classification of Portuguese dialects (Nova proposta de classificação dos dialectos galego-portugueses, 1964–1971) that has become a well-known reference in Portuguese studies. This dialectal analysis was based on his personal experience as a field researcher for the Atlas Lingüístico de la Península Ibérica (ALPI) during the 1950s. However, a close examination of the maps contained in the only published volume of the ALPI (1962) reveals some discrepancies with Cintra’s linguistic classification, and these differences are confirmed by the unpublished materials of the Atlas Linguístico-Etnográfico de Portugal e da Galiza (ALEPG). Therefore, it is imperative to assess the internal consistency of the Nova proposta. With this aim in mind, this paper undertakes a quantitative analysis of almost 50.000 forms extracted from the unpublished materials of the ALPI in order to examine the geographic distribution and frequency of use of the five phenomena used by Cintra to establish his proposal. The utilization of a sizable corpus and quantitative cartography provides a more dynamic picture of the dissemination of linguistic change and the vitality of dialectal features. This analysis reveals several inconsistencies in Cintra’s proposal, due to the limitations of traditional dialectology and an opaque data selection.