We report cases of 2 patients with pure motor Guillain-Barré syndrome of explosive onset who required mechanical ventilation for more than 2 months. Their electrophysiologic findings and poor clinical recoveries suggested severe axonal degeneration involving the motor nerves. Enzyme-linked immunosorbent assay and thin-layer chromatogram-immunostaining showed the sera of both patients had high IgG antibody titer against GD1a ganglioside. Their titers decreased with the clinical course of the illness. GD1a as well as GM1, appears to be the target pathogenic antigen in motor axon disorders. Elevated IgG anti-GD1a antibody titer may prove useful for predicting severe GBS.