Plasma exchange is contraindicated in 10 to 20% of patients with Guillain-Barré syndrome (GBS). The optimal schedule for intravenous immune globulin (IVIg) therapy has not yet been established in these patients. The objective was to compare the efficacy and safety of two IVIg treatment durations in patients with GBS with contraindications for plasma exchange. In this randomised, double blind, multicentre phase II trial conducted in seven French centres, patients with GBS with severe haemostasis, unstable haemodynamics, or uncontrolled sepsis were randomly assigned to 0.4 g/kg/day IVIg for 3 or 6 days. The primary outcome measure was the time needed to regain the ability to walk with assistance. Thirty nine patients were included from March 1994 to May 1997, 21 in the 3 day group and 18 in the 6 day group. Time to walking with assistance was non-significantly shorter in the 6 day group (84 (23-121) v 131 days (51-210), p=0.08); the difference was significant in ventilated patients (86 days (13-151) in the 6 day group v 152 days (54-332) in the 3 day group; p=0.04). The prevalence and severity of IVIg related adverse effects were comparable between the two groups. In conclusion, in patients with GBS and contraindications for plasma exchange, especially those who need ventilatory assistance, IVIg (0.4 g/kg/day) may be more beneficial when given for 6 days rather than 3 days.