The underlying causes of type I congenital disorders of glycosylation (CDG I) have been shown to be mutations in genes encoding proteins involved in the biosynthesis of the dolichyl-linked oligosaccharide (Glc(3)Man(9)GlcNAc(2)-PP-dolichyl) that is required for protein glycosylation. Here we describe a CDG I patient displaying gastrointestinal problems but no central nervous system deficits. Fibroblasts from this patient accumulate mainly Man(9)GlcNAc(2)-PP-dolichyl, but in the presence of castanospermine, an endoplasmic reticulum glucosidase inhibitor Glc(1)Man(9)GlcNAc(2)-PP-dolichyl predominates, suggesting inefficient addition of the second glucose residue onto lipid-linked oligosaccharide. Northern blot analysis revealed the cells from the patient to possess only 10-20% normal amounts of mRNA encoding the enzyme, dolichyl-P-glucose:Glc(1)Man(9)GlcNAc(2)-PP-dolichyl alpha3-glucosyltransferase (hALG8p), which catalyzes this reaction. Sequencing of hALG8 genomic DNA revealed exon 4 to contain a base deletion in one allele and a base insertion in the other. Both mutations give rise to premature stop codons predicted to generate severely truncated proteins, but because the translation inhibitor emetine was shown to stabilize the hALG8 mRNA from the patient to normal levels, it is likely that both transcripts undergo nonsense-mediated mRNA decay. As the cells from the patient were successfully complemented with wild type hALG8 cDNA, we conclude that these mutations are the underlying cause of this new CDG I subtype that we propose be called CDG Ih.