Objective: Chronic rhinosinusitis (CRS) is prevalent and causes a great negative impact on quality of life. Primary antibody deficiency (PAD) is highly prevalent with CRS compared to background population. There are efficient treatment options to consider when CRS is a symptom of PAD. The condition seems to be underdiagnosed. Design: A prospective cohort-study investigating adults with CRS following the EPOS 2020 criteria for possible PAD. Results: 138 patients were included in the study. Mean age was 49 years. Twenty-nine (21%) had hypogammaglobulinemia. Nine of 83 (11%) patients had insufficient response to pneumovax polysaccharide vaccine, but insufficient clinical history for diagnosis of Specific Antibody Deficiency (SPAD). Four patients met the clinical criteria for PAD; one had Common Variable Immune Deficiency (CVID), one had IgA deficiency and two had IgG subclass deficiency. No clinical characteristic was predictive of PAD and severity of CRS was not indicative of hypogammaglobulinemia or insufficient vaccine response. Conclusion: Immunoglobulin testing should be a routine part of the diagnostic work-up in chronic rhinosinusitis before considering biologics, as primary antibody deficiency is an under-recognized but treatable cause of refractory disease. Keywords