OBJECTIVE: To study bacteria and viruses in maxillary sinuses of patients with primary hypogammaglobulinemia receiving immunoglobulin therapy. DESIGN: Prospective cross-sectional study during 6 months. SETTING: Tertiary care university hospital. PATIENTS: Seventeen patients with primary hypogammaglobulinemia (10 males and 7 females; mean age, 39 years [age range, 11-71 years]). Sixteen patients had common variable immunodeficiency, and 1 patient had X-linked agammaglobulinemia. MAIN OUTCOME MEASURES: Magnetic resonance imaging and x-ray imaging of paranasal sinuses when patients did not have signs of acute infection and reevaluation 6 months later. Maxillary sinus aspiration and lavage were performed at a follow-up visit. Sinus fluid analysis for bacteria and viruses was performed by culture and by polymerase chain reaction. A questionnaire on symptoms related to sinusitis was administered during the follow-up period. RESULTS: Among 17 patients, 9 (53%) had radiologically defined sinusitis without subjective symptoms at study enrollment. At reevaluation 6 months later, radiological findings remained unchanged in two thirds of the patients. Among 15 patients, bacteria were found in sinus lavage samples from 13 patients, and viruses were found in samples from 7 patients. Eight patients had 2 pathogens or more on bacterial culture. Rhinovirus was identified from sinus lavage samples in 5 patients (33%), enterovirus in 3 patients (20%), and respiratory syncytial virus in 1 patient (7%). Pathogenic bacteria were found in maxillary sinuses of all patients who tested positive for rhinovirus and enterovirus. No fungi were found. During the follow-up period, 6 patients reported mucopurulent drainage. CONCLUSIONS: Bacteria and viruses were commonly found in maxillary sinuses of patients with primary hypogammaglobulinemia. Yearly evaluation by an ear, nose, and throat surgeon is recommended.