An Old Order Mennonite child was evaluated for gross motor delay, truncal ataxia, and slow linear growth. The diagnostic evaluation, which included sub-specialty consultations, neuroimaging, and metabolic testing, was long, costly, and did not yield a diagnosis. Recognition of a similarly affected second cousin prompted a genome-wide homozygosity mapping study using high-density single nucleotide polymorphism (SNP) arrays. SNP genotypes from two affected individuals and their parents were used to localize the disease locus to a 14.9 Mb region on chromosome 6. This region contained 55 genes, including SLC17A5, the gene encoding the lysosomal N-acetylneuraminic acid transport protein. Direct sequencing of SLC17A5 in the proband revealed homozygosity for the 115C --> T (R39C) sequence variant, the common cause of Salla disease in Finland. Three additional affected Mennonite individuals, ages 8 months to 50 years, were subsequently identified by directed molecular genetic testing. This small-scale mapping study was rapid, inexpensive, and analytically simple. In families with shared genetic heritage, genome-wide SNP arrays with relatively high marker density allow disease gene mapping studies to be incorporated into routine diagnostic evaluations.