Brain-derived neurotrophic factor (BDNF) has the theoretical potential to protect neurones from axonal degeneration. The objective of this study was to discover whether brain-derived neurotrophic factor is safe in Guillain-Barré syndrome, and to make preliminary observations of its efficacy. This was a parallel group randomized controlled trial of subcutaneous brain-derived neurotrophic factor 25 microg/kg daily compared with placebo for up to 24 weeks or until patients could walk without aid. Six patients received brain-derived neurotrophic factor, of whom three had serious adverse events including one death. Four patients received placebo, of whom two had serious adverse events including one death. The rate and extent of recovery were similar in the two groups. This pilot study did not detect any serious adverse events attributed to brain-derived neurotrophic factor treatment.