Introduction: Worldwide, polyvalent immunoglobulin (Ig) use is rising. Together with the limited supply, this puts pressure on Ig availability. A clear overview on a country’s usage pattern helps in forecasting future needs. This research aims to provide an overview of Ig use in Belgium on the different indications, including an estimation of off-label use. Methods: Multiple data sources were used. Existing claims data were explored for reimbursed Ig use between 2010 and 2018. General 2018 sales data from the firms were compared to the reimbursed use to serve as a proxy for off-label use. Indication-specific information was retrieved via a proxy: diagnostic codes available during day-care and inpatient hospitalization. Results: In 2018, 7,556 patients had reimbursed Ig. The most prevalent indication, both in terms of patient numbers and volume, was primary immunodeficiency (PID). In Belgian hospitals, the currently reimbursed indications represented 84.4% of patients (PID [≈35%], secondary immunodeficiency [SID] [≈21%], primary immune thrombocytopenia [≈10%], chronic inflammatory demyelinating neuropathy [CIDP] [≈8%], Guillain-Barre syndrome [≈6%], Kawasaki [≈2%], streptococcal toxic shock [≈2%] and multiple motor neuropathy [≈1%]), and 82.4% of Ig use (predominantly PID [≈33%] and CIDP [≈21%]). Although no direct data on off-label use were available, crude estimates derived from indirect sources showed a proportion of around 15.4%. Conclusion: Our research offers the first comprehensive overview on Ig use in Belgium, including a detailed description of reimbursed use, as well as approximations to off-label use. In view of increasing pressure on Ig availability, better understanding Ig needs and trends, would benefit from an effective indication-specific national registry system (ideally covering both reimbursed and nonreimbursed use).