Carbohydrate-deficient glycoprotein (CDG) syndromes are genetic multisystemic disorders characterized by defective N-glycosylation of serum and cellular proteins. The activity of phosphomannomutase was markedly deficient (⩽ 10% of the control activity) in fibroblasts, liver and/or leucocytes of 6 patients with CDG syndrome type I. Other enzymes involved in the conversion of glucose to mannose 1-phosphate, as well as phosphoglucomutase, had normal activities. Phosphomannomutase activity was normal in fibroblasts of 2 patients with CDG syndrome type II. Since this enzyme provides the mannose 1-phosphate required for the initial steps of protein glycosylation, it is concluded that phosphomannomutase deficiency, which is first reported here for higher organisms, is a cause, and most likely the major one, of CDG syndrome type I.