Patients with chronic inflammatory demyelinating polyneuropathy (CIDP) received immune globulin intravenous, 10% caprylate/chromatography purified (IGIV-C, Gamunex; n=59) or placebo (n=58) every 3 weeks for up to 24 weeks (first period) in a randomized, double-blind, parallel-group, response-conditional, crossover study. Motor and sensory nerves were assessed at baseline and endpoint/week 24. A nonsignificant trend toward improvement in the proximal amplitude of the most severely affected motor nerve was observed with IGIV-C (0.69+/-1.86 mV) versus placebo (0.47+/-2.29 mV), and a greater improvement of 1.08+/-2.15 mV with IGIV-C versus 0.46+/-2.03 mV with placebo (P=0.089) was observed with exclusion of data from Erb's point stimulation. Greater improvements from baseline favoring IGIV-C were observed for 127/142 electrophysiologic parameters. The averaged motor amplitudes from all motor nerves significantly improved with IGIV-C versus placebo [treatment difference, 0.62 mV; 95% confidence interval (CI), 0.05, 1.20; P=0.035], and conduction block decreased significantly (treatment difference, -5.54%; 95% CI, -10.43, -0.64; P=0.027), particularly in the lower limbs. Overall, the data suggest that IGIV-C improves electrophysiologic parameters in CIDP.