Machado-Joseph disease (MJD) is an autosomal, dominantly inherited neurodegenerative disorder of adult onset. The prevalence of MJD reaches its highest values in the islands of the Azores. A research program was undertaken to study the origin and spread of the mutant gene in the Azorean populations. Here, we present the first results of such a study. The Azorean MJD patients are grouped in 34 families and are distributed on 4 of the 9 Azore islands. Values of prevalence, carrier rate, and number of individuals at risk are reported. The genealogies of the patients were reconstructed to identify the founders. An analysis of the geographic distribution of the birthplaces of the patients compared with the birthplaces of the founders revealed the existence of areas that are clusters for both, thus defining crucial sites for the origin of the disease. Preliminary results on the number of links between the affected families show that 64.7% of them have at least one link with another MJD family. So far, a single source for the introduction of the mutant gene in the Azores has not been identified.