Glycogen storage disease type II (GSDII) is an autosomal recessive disorder resulting from inherited deficiency of the enzyme lysosomal acid alpha-glucosidase. Over 40 different mutations have been described but no large deletions have been previously identified. We now describe a homozygous large (9-kb) deletion extending from IVS 15 to 4 kb downstream of the terminal exon (exon 20), detected by polymerase chain reaction (PCR)-based methods. The deletion was initially suspected because of failure to amplify a contiguous group of exons by PCR. We hypothesized an Alu/Alu recombination, based on our prior demonstration by Southern blotting of Alu elements in the regions potentially flanking the deletion. Additional sequence analysis of genomic fragments confirmed the presence of Alu elements and allowed the design of flanking primers for PCR amplification. Amplification resulted in a smaller than normal fragment (0.7 vs. 10 kb) in homozygosity in the proband and in heterozygosity in her parents. Cloning and sequencing of the smaller than normal 0.7-kb deletion fragment revealed an Alu/Alu deletion junction. In heterozygosity this deletion would not be detected by currently standard PCR mutation detection methods. Based on other Alu-mediated deletions, this deletion is likely to be recurrent and should be screened for in all non-consanguineous GSDII patients, particularly when only one mutation has been identified and none of the 12 single-nucleotide polymorphisms in the deleted region are heterozygous. These observations also suggest that initial characterization of genes at disease-causing loci should include a search for Alu and other repetitive elements to facilitate subsequent PCR-based mutation analysis.