The increased use of intravenous immunoglobulins (IVIg) in the treatment of neurological autoimmune diseases has led to more awareness of adverse reactions. We studied prospectively the side effects of IVIg during 84 treatment courses with a total of 341 infusions under routine clinical conditions. Mild reactions were common. Headache was noted most often, occurring during 30% of treatment courses. There were three severe adverse events (3.6% of all treatment courses) that led to discontinuation of the treatment, namely thrombosis of the jugular vein, allergic reaction and retrosternal pressure. Significant changes in laboratory findings were seen for leucocytes, erythrocytes, haematocrit, haemoglobin, ALAT and ASAT. None of these changes were clinically relevant. The elevation of liver enzymes was dependent on the IVIg preparation used, while there was no association with the underlying disease, age, or gender of the patient. In conclusion, this prospective study confirms the high frequency of mild, self-limited side effects of IVIg. Elevation of liver enzymes may possibly be associated with certain IVIg preparations. Bearing these complications in mind, this prospective study supports the notion that IVIg can generally be regarded as safe, leading to severe adverse events during only 3 (0.9%) of 341 infusions (or 3 of 84 treatment courses, 3.6 %). However, careful monitoring for severe side effects remains mandatory, and we propose that laboratory findings like full blood count, renal and liver function should be monitored routinely.