Background Different preparations of intravenous immunoglobulin (IVIg) are considered to have comparable clinical efficacy but this has never been formally investigated. Some patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) report that some IVIg brands are more effective than others. A liquid IVIg preparation is more user friendly and potentially can be infused at a faster rate. Objectives The primary objective was to compare the efficacy of two different IVIg brands in CIDP. The secondary objective was to compare their safety. Methods This was an investigator-initiated multi-centre randomised controlled double-blind trial. Twenty-seven patients with active but stable CIDP treated with their individual stable IVIg (Gammagard S/D) maintenance dose and interval were randomised to receive four infusions of freeze-dried 5% IVIg (Gammagard S/D) or the new liquid 10% IVIg (Kiovig). The overall disability sum score (ODSS) was used as the primary outcome scale. The equivalence margin was defined as a difference of <= 1 point in mean Delta ODSS between treatment groups. Main secondary outcome scales were the MRC sum score and the Vigorimeter. Results Repeated measurements analysis of variance, adjusted for baseline ODSS, showed a clinically insignificant treatment difference of 0.004 (95% CI -0.4 to 0.4). We also found no significant differences in any of the other outcome measures. Besides a lower occurrence of cold shivers in patients randomised to Kiovig (p=0.03), no significant differences were found in the occurrence of adverse events. Conclusions This trial demonstrated equal clinical efficacy between a freeze-dried and a liquid IVIg preparation for maintenance treatment of CIDP.