Replacement therapy with immunoglobulin G (IgG) given as intravenous or subcutaneous (SC) infusions is the standard treatment for patients with primary immunodeficiency. Due to the life-long need for replacement, increased flexibility in the administration and dosage regimens would improve patients' quality of life. A population pharmacokinetic model that can predict plasma IgG concentrations for various routes, dosage regimens, and patient groups is a valuable tool to improve patient therapy. Such a model was developed based on IgG concentrations from 151 unique adult and pediatric patients who participated in 4 clinical trials of intravenous and SC IgG replacement therapy. Simulations predicted that the same total IgG dose, delivered SC, either in 1 biweekly dose (once every 2 weeks), or in 2 weekly doses, results in IgG peak and trough concentrations that remain within ± 10% of each other throughout the 14-day period. The developed population pharmacokinetic model predicted that biweekly SC Hizentra dosing offers a viable alternative to weekly SC therapy, allowing more flexible and optimized dosage regimens for patients with primary immunodeficiency.