Recently, a subset of fucosidosis patients was identified in which the single EcoRI site in the open reading frame of the human cDNA encoding alpha-L-fucosidase was obliterated. We have employed the polymerase chain reaction technique to amplify alpha-L-fucosidase DNA from the five patients known to carry the EcoRI abnormality as well as four patients and two additional fucosidosis patients who do not carry the EcoRI abnormality. Sequence analysis of the amplified DNA has determined that the EcoRI site was destroyed by a C-T transition in the last position of the EcoRI site. This single base change results in the generation of a stop codon 120 base pairs upstream of the normal stop codon. In addition, we have determined that EcoRI cleavage of amplified DNA may be a useful diagnostic tool in the diagnosis of heterozygotes and in prenatal diagnosis of fetuses at risk for this disease.