Background: ICI pneumonitis is an immune-related adverse event (irAE) of the lung and often requires discontinuation of ICI. While some pneumonitis cases resolve within the recommended 4-6-week period of corticosteroid therapy, others either fail to improve or have multiple flares necessitating longer or repeated courses of steroids. We investigated the clinical features and courses of pts with chronic pneumonitis. Methods: We analyzed 869 pts with NSCLC initiated on ICIs at our institution between 2011 and 2019 for development of ICI pneumonitis. Chronic pneumonitis was defined as any pneumonitis requiring a total of ≥12 weeks of steroids, given either continuously or over multiple courses of treatment. Cases of chronic pneumonitis in which the initial course of steroids lasted ≥12 weeks without interruption were termed “primary refractory pneumonitis”. Subsequent episodes of pneumonitis were categorized as either “recurrent pneumonitis” from ICI rechallenge or “pneumonitis flare” after steroid taper without ICI rechallenge. Chest CT scans were analyzed to classify the imaging patterns. Results: Of the 869 pts analyzed, 44 developed ICI pneumonitis (5.1%) and 22 of the 44 (50%) experienced chronic pneumonitis (Grade 2 in 11, Grade 3 in 9, and Grade 4 in 2). A cryptogenic organizing pneumonia (COP) pattern was the most common CT pattern among all pneumonitis cases (30/44) and in chronic pneumonitis cases (14/22). Among chronic pneumonitis cases, the median number of total weeks on corticosteroid therapy was 25.9 (range: 12.4 – 114.4 weeks). Four pts required additional immunosuppressive agents including mycophenolate or infliximab. Fourteen of 22 pts with chronic pneumonitis had primary refractory pneumonitis, while the remaining 8 pts were weaned off of steroids within 12 weeks but later developed additional episode(s) of pneumonitis, ultimately resulting in a total steroid duration of ≥12 weeks. The 14 pts with primary refractory pneumonitis had significantly shorter time to pneumonitis onset compared to the other 8 pts (median time to onset: 1.8 vs. 5.5 months, Wilcoxon ranksum p = 0.04). Seventeen of 22 patients had their ICI permanently discontinued; of these, 9 pts subsequently experienced pneumonitis flare after steroid taper, necessitating additional course(s) of steroid therapy. 5 of the 22 patients were rechallenged with ICI, and 4 of them had recurrent pneumonitis with ICI rechallenge. Conclusions: Half of the pts diagnosed with pneumonitis developed chronic pneumonitis requiring at least 12 weeks in aggregate of glucocorticoid therapy. Some patients had an initial prolonged steroid course while others initially improved and then flared, even after ICI discontinuation. Recognition of chronic pneumonitis as a distinct and common clinical entity is important in management of pts with ICI pneumonitis.