Microsatellites can be highly unstable and show a high level of polymorphism between individuals. Here we present the analysis of the CAG trinucleotide repeat polymorphism at the SBMA locus in 57 phenotypically normal individuals rigorously assigned to the Spanish Basque population. Results are compared with 100 Spanish non-Basque individuals who were already analyzed by us (175 alleles). This is the first study undertaken in these populations for this marker. In addition, we compared our results with those published for other populations. Relative allele frequencies showed differences between the samples and no unimodal distribution. The expected heterozygosity in the Basque sample was slightly lower than in the non-Basque sample. Conformity with Hardy-Weinberg equilibrium was verified by three tests. When compared with published data, the predominant alleles appear to be the same in the various populations. There are more differences between Basques and other Caucasoid samples than between non-Basques and Caucasoid samples. Population relationships were also examined by dendrograms based on genetic distances. The results obtained showed some peculiarities in the Basque population. The high degree of similarity with other dendrograms based on different markers and the efficiency of this STR marker in differentiating closely related populations, support the potential usefulness of microsatellites as tools for human population studies.