Glucose-galactose malabsorption (GGM) is an autosomal recessive disease with life-threatening newborn diarrhea caused by mutations in the Na(+) /glucose cotransporter gene SLC5A1. Because of its rarity, the clinical course of the disease has not been well studied. Here, we report 33 patients with GGM from a large Old Order Amish pedigree and the associated mutations in SLC5A1 gene. Clinically, all affected individuals presented with classic watery diarrhea and dehydration. The increased bowel sounds, distended abdomen, vigorous nursing regardless of their illness, and irritability and apathy were also noted as part of the initial presentation. Patients underwent a dramatic turnaround with an immediate cease of the diarrhea and a quick rehydration if they were correctly diagnosed and adequately managed, followed by a normal growth and development pattern afterwards; whereas a prolonged clinical course would follow if the disease was not recognized. Sequence analysis of the 15 protein-coding exons and the corresponding exon-intron boundaries of SLC5A1 gene revealed four homozygous missense mutations, c.152A>G (p.N51S), c.1231G>A (p.A411T), c.1673G>A (p.R558H), and c.1845C>G (p.H615Q), that co-segregate with the GGM phenotype in all of the affected individuals. These findings suggest that founder effect of the SLC5A1 mutations associated with the disease in Amish and a population specific genetic testing is in need to pursue an early diagnosis which is critical for a favorable outcome.