This study consisted of a 12-week wash-in/wash-out period followed by a 28-week efficacy period. During the 28-week efficacy period, subjects visited the study site at least every 4 weeks for efficacy and safety evaluations and additionally recorded details regarding IgPro20 dose and certain aspects of efficacy and safety in a diary. Pharmacokinetic (PK) parameters were assessed in a sub-group of subjects during 1 treatment interval at steady-state (Week 28 ± 1).