SummaryThis open-label multi-centre study evaluated Gammaplex V R 5%, a human intravenous immunoglobulin (IVIG) 5% liquid, in 25 children and adolescent patients (aged 3-16 years) with primary immunodeficiency diseases (PIDs). Subjects received Gammaplex 5% (at doses of 300-800 mg/kg/infusion) for 12 months, with a 3-month follow-up. The primary efficacy end-point was the incidence of serious acute bacterial infections (SABIs) during the 12-month treatment period. Secondary objectives assessed safety and tolerability. Nineteen males and six females were treated using the same infusion schedule as their prior IVIG treatment (14 and 11 subjects on 21-and 28-day dosing schedules, respectively). Two SABIs of pneumonia were reported, resulting in an annual SABI event rate of 0Á09 [upper one-sided 99% confidence interval (CI) 5 0Á36]. Twenty-one subjects (84%) experienced ! 1 infection during the study, with a median infective episode per subject/year of 3Á08 (range 5 0-10Á4). Sixteen subjects (64%) missed ! 1 day of nursery or school because of infection or other illness. All trough immunoglobulin G levels exceeded 7Á00 g/l after 15 weeks (mean 5 9Á69 g/l; range 5 7Á04-15Á35 g/l). Product-related adverse events occurred in 14 subjects (56%); none were serious. Of 368 total infusions, 97 (26%) were associated temporally with an adverse event ( 72 h after infusion), regardless of causality. Laboratory test results and adverse-reaction data showed no evidence of product-related haemolysis or thromboembolic events. These data demonstrate that Gammaplex 5% is effective in preventing SABIs and well tolerated in children and adolescents with PID.