In fibroblasts from five patients with carbohydrate-deficient glycoprotein syndrome type 1, the incorporation of [2-3H] mannose into mannose phosphates, GDP-mannose, GDP-fucose, dolichol-P-mannose, lipid-linked oligosaccharides, and glycoprotein fraction was determined. We observed a 3- to 5-fold reduction of incorporation of radioactivity into mannose 1-phosphate, GDP-mannose, GDP-fucose, dolichol-P-mannose, and nascent glycoproteins. The incorporation of radioactivity into mannose 6-phosphate was normal. The formation of lipid linked oligosaccharides was only slightly affected (</=20%), but their size was severely reduced, mostly containing five or fewer residues. As a consequence, truncated oligosaccharides were transferred to newly synthesized glycoproteins. The metabolic changes can be explained by a deficiency of phosphomannomutase activity, which was reduced to </=10% of control.