We have analysed allele distribution at the highly polymorphic variable number of tandem repeats (VNTR) locus D1S80 (pMCT118) in the Polish population using the polymerase chain reaction (PCR) technique. Characteristics of the D1S80 locus makes it a very useful marker for population genetic research, genetic linkage studies and forensic identification of individuals. During our routine application of the D1S80 marker to paternity testing in several cases of homozygosity detected by polyacrylamide gel electrophoresis, heteroduplex formation for alleles 18 and 24 was also observed. Direct sequencing of PCR products revealed that alleles 18 and 24 of locus D1S80 actually represent a mixture composed of different sequences. Our observations indicate that identification of some 18 and 24 VNTR alleles based only on size estimated in electrophoretic analyses could lead to errors in paternity testing and DNA profiling.