Justification: Chronic rhinosinusitis (CRS) is one of the most common inflammatory diseases with an incidence and prevalence superior to 10%. Unfortunately, more than 30% of patients do not respond to standard medical and surgical treatment, thus continuously increasing the symptomatologic and socio-economic burden of this disease.

Hypothesis: The investigators believe that the addition of azithromycin (AZI) to the treatment regimen of patients with refractory CRS failing conventional medico-surgical treatment will be beneficial in a symptomatologic and endoscopic level.

Primary objective: 1- To evaluate whether Azithromycine 250 mg PO three times weekly is effective in controlling signs and symptoms of CRS in high-risk patients unresponsive to standard management after endoscopic sinus surgery (ESS) with budesonide irrigations.

Secondary objectives:

i) Validate a simple and concise treatment algorithm for patients refractory to standard CRS treatment of ESS and BUDI, with the addition of low-dose AZI.

ii) Characterise and define the population deemed "high-risk" for standard CRS treatment failure by evaluating: 1) demographics, 2) inflammatory states and 3) the nasal flora microbiome of patients at the different follow-up points of this study.

iii) Explore the mechanisms of AZI by assessing the changes in inflammatory states and the nasal flora microbiome associated with successful AZI therapy.

Methods: Inclusion of all patients admitted for endoscopic sinus surgery (ESS) for CRS operated by the same surgeon (MD). Following their first postoperative visit (2 weeks), all patients will receive nasal irrigations with budesonide (BUDI) twice daily for 4 months and will be re-evaluated. If there is a failure of treatment, patients eligible to receive AZI will be randomized in to two groups, AZI 250mg or a placebo three times a week for 4 months. At every follow-up, complete endoscopic exams will be performed, along with sinus cultures and brush cytology.

Population: All patients deemed "high-risk" with CRS admitted for ESS between October 2014 and October 2015.

Inclusion Criteria:

Patients with ≥1 of the following criteria:

• history of sinus surgery,
• first sinus surgery at ≤38 years of age,
• an absolute eosinophilia of ≥500 cells/mm,
• serum IgE levels of >150 kIU/L,
• a Gram negative bacteria in a sinus culture,
• the presence of intra-operative eosinophilic mucin.

Exclusion Criteria:

• Patients with cystic fibrosis, inverted papillomas, osteomata, mucoceles or other lesions of the base of skull will be excluded.
• Patients with an elevated cardiovascular disease risk will be excluded from the randomized clinical trial part of this study.