In this article we describe the molecular characterization of 104 independent alpha-thalassemia patients identified by hematological analysis and family studies. During the study, another six chromosomes were identified with rearrangements of the alpha-cluster or point mutations in the alpha 2-globin gene, not associated with alpha-thalassemia, in healthy relatives of the patients. The molecular defects were established by Southern blot analysis and, if no deletions could be identified, the alpha-globin genes were investigated by denaturing gradient gel electrophoresis and single strand conformation analysis for the presence of point mutations. Following this strategy, we were able to identify the molecular basis of 131 independent alpha-thalassemia chromosomes. In two individuals, the alpha-thalassemia determinant could not be demonstrated at the molecular level. We identified eight different deletion and five non-deletion alpha-thalassemias, three rearrangements in the alpha-cluster, two alpha-chain variants, and a silent mutation in the alpha 2-globin gene not associated with alpha-thalassemia. The large heterogeneity of alpha-thalassemia mutations seen in the Dutch population might be typical for northern European countries where, besides the more common mutations introduced by migration, a variety of sporadic mutations was also found in the autochthonous population. The screening strategy as described here, capable of identifying a wide spectrum of both deletions and point mutations, identified 98% of the alpha-thalassemia determinants present in 133 chromosomes.