The congenital disorder of glycosylation type Ia (CDG-Ia) presents a broad clinical spectrum. Some patients suffer from acute vascular events (thrombosis and bleeding) and stroke-like events. No correlations have been made between the marked hemostasis abnormalities of CDG-Ia and the occurrence of acute vascular events. We report on 6 patients with CDG-Ia presenting vascular events, then we analyze the clinical and hemostasis data of 39 CDG-Ia patients described in the literature, 17 with vascular events (E) and 21 unscathed from any event (EF), to determine the risk factors for acute vascular events in CDG-Ia. Acute vascular events occurred in patients younger than 15 years, especially with fever and prolonged immobilization. Hemostasis and liver cytolysis were statistically abnormal in patients younger than 5 years whatever the occurrence of vascular events and they normalized with time. Higher factors VIII and IX activities were statistically observed in the E cluster (p=0.03) compared to the EF cluster. The activity/antigenicity ratio for protein C (p=0.02) was also higher in the E group. CDG-Ia patients younger than 15 years old are at risk of acute vascular events. The paradoxical results-abnormal VIII and IX factors in EF patients and normal results in E patients, while XI, antithrombin, protein C, ASAT and ALAT are abnormal in both groups, could suggest a disequilibrium between prothrombotic and antithrombotic factors in the E group. Vascular events may also occur in patients where glycoproteins are proportionally more hypoglycosylated, particularly protein C.