A novel method of haplotype identification is discussed allowing simultaneous detection of two adjacent polymorphic sites, a single nucleotide polymorphism (SNP) and a length polymorphism within 1-2 kilobase distance. The method combines allele specific-amplification with high-throughput, automated ultrathin-layer gel electrophoresis analysis of fragment size polymorphism. A typical application is shown for genotyping the -521 C/T single nucleotide polymorphism and the 120 bp duplication in the 5"-upstream region of the dopamine D4 receptor (DRD4) gene. We have also demonstrated that the haplotypes of double heterozygotes for -521 C/T and for the 120 bp duplication can be clearly distinguished, that has only been possible previously by extensive pedigree analysis.