Many psychiatric disorders are influenced by genetic factors, but the genetic components of complex diseases may not follow clear inheritance patterns. Although the patients may share a common clinical phenotype, the cause of the syndrome may consist of a heterogeneous collection of both genetic and/or environmental components. One method to minimize genetic heterogeneity in studies of complex disorders is to select a very homogenous study population. The average number of families with the same last name, when corrected for population size, is an excellent marker for the degree of homogeneity. We used surname analysis to evaluate the homogeneity of the Portuguese population of Madeira, comparing it with previous data on the homogeneity of populations of mainland Portugal, the Azores, and both rural and urban US populations. The average number of families with the same last name corrected for population size was 33.84 in Madeira, 30.88 in the Azores, and 21.42 in Coimbra (mainland Portugal) compared with 1.13 in rural and 0.38 in urban United States. This surname analysis supports the premise that the Portuguese population is a highly homogenous population, with the highest homogeneity in Madeira and the Azores, making it a good study population for molecular genetic analyses.