Thrombotic events are an increasingly recognized complication of treatment with intravenous immunoglobulins (IVIg). We aimed to define clinical characteristics, risk factors and outcome for venous thrombosis as opposed to arterial thrombosis following administration of IVIg. Six patients with post-IVIg venous thrombosis were identified at our institution. In addition, a review of the literature revealed 65 reported cases. Arterial thrombosis (stroke and myocardial infarction) was four times more common than venous thrombosis (deep vein thrombosis and pulmonary embolism). The incidence rate was estimated at 0.15-1.2% per treatment course, but the large increase in reported cases in 2003 suggests that the true incidence may be significantly greater. The following differences were found between arterial and venous events: arterial thrombosis occurred early after IVIg administration (49% within 4 h, 77% within 24 h) and was associated with advanced age and atherosclerotic vascular disease; venous thrombosis occurred later (54% more than 24 h after IVIg administration) and was associated with factors contributing to venous stasis (obesity and immobility). Thirteen patients died (mortality 20%), 11 of whom had arterial thrombosis. In conclusion, IVIg-associated thrombosis is more common than previously recognized, and is associated with significant mortality. The different characteristics of arterial and venous events may reflect different pathophysiological mechanisms. A better understanding of these mechanisms should aid in defining a risk-benefit ratio for the individual patient.