Importance: Eosinophilic chronic rhinosinusitis (eCRS), contemporarily classified as diffuse type 2 dominant chronic rhinosinusitis (CRS), is characterized by eosinophil-dominant mucosal inflammation. Contemporary management of eCRS as an inflammatory airway condition is multimodal with corticosteroid irrigations after the surgical creation of a neosinus cavity. Objectives: To assess long-term treatment outcomes in patients with primary diffuse type 2 CRS or eCRS receiving multimodal treatment. Design, Setting, and Participants: A prospective cohort study of patients seen in a tertiary rhinology practice recruited from May 2010 to November 2018 was conducted. Follow-up duration was 12 months or more following endoscopic sinus surgery (ESS) with a neosinus cavity formed. Data analysis was performed from August to November 2020. Consecutive adult (≥18 years) patients diagnosed with primary diffuse type 2 dominant CRS or eCRS based on the European Position Paper on Rhinosinusitis and Nasal Polyps 2020 criteria were included. Type 2 inflammation was defined as more than 10 eosinophils per high-power field obtained from sinus mucosal biopsy and managed with neosinus cavity ESS and ongoing corticosteroid irrigations. Exclusion criteria were less than 12 months of follow-up and secondary CRS. Interventions: Endoscopic sinus surgery with complete removal of intersinus bony partitions to create a neosinus cavity. Nasal irrigation (240 mL) with betamethasone, 1 mg, or budesonide, 1 mg, daily for 3 to 6 months after ESS and tapered to an as-needed basis (minimum, 2-3 per week). Main Outcomes and Measures: Poor control was defined as polyp recurrence (polyp growth in >1 sinus area on a single side), use of long-term systemic therapy (biologic therapy or ≥3 consecutive months of oral corticosteroids), and revision surgery involving polypectomy. The disease in patients with no poor control criteria was defined as well controlled, and the disease in those with 1 or more criteria was considered poorly controlled. Maintenance medical therapy use and patient-reported outcomes based on the 22-item Sinonasal Outcomes Test for preoperative and last follow-up were collected. Results: Of the 222 participants recruited with primary diffuse type 2 dominant CRS or eCRS and minimum of year of follow-up, 126 were men (56.8%). Mean (SD) age was 54.8 (13.6) years, and median (SD) follow-up was 2.2 (2.2) years. Of the 222 patients, 195 (87.8%) had well-controlled disease, 16 (7.2%) had polyp recurrence, 7 (3.2%) continued to receive long-term oral corticosteroid therapy, 5 (2.3%) received biologic therapy, and 8 (3.6%) underwent a revision polypectomy. Clinically meaningful change on the 22-item Sinonasal Outcomes Test and the nasal subdomain score was maintained at the last follow-up in 134 patients (67.0%). Poor disease control was not associated with poor adherence to irrigation use. Conclusions and Relevance: The findings of this cohort study suggest that long-term disease control and reduction in symptom burden in patients with primary diffuse type 2 CRS or eCRS might be achieved when managed as an inflammatory disorder. Maintenance corticosteroid irrigations in the population examined appeared to be successfully self-tapered to disease activity.