Background: Castleman disease (CD) has three subtypes: Unicentric (UCD), Human herpesvirus-8 associated multicentric (HHV-8 MCD) and idiopathic multicentric (iMCD). Outcomes for patients with iMCD are poor and treatment options are limited, with only one FDA-approved therapy (siltuximab in April 2014). Further, the lack of CD-specific ICD codes until 2017 has limited real-world evaluation. We identified iMCD patients in an electronic health record (EHR)-derived dataset and described their clinical characteristics, treatment patterns, and real-world overall survival (rwOS). Methods: Patients with a possible diagnosis of CD as of 8/31/20 were identified from the nationwide deidentified Flatiron Health EHR-derived database using patient-level structured data (e.g., ICD-9/10 codes) and unstructured data (e.g., clinician notes), curated via technology-enabled manual abstraction to confirm CD diagnosis and treatments received. Descriptive statistics summarized patient characteristics and treatment patterns. Patients without structured data within 90 days after diagnosis were excluded from treatment patterns analyses. 5-year rwOS rate was estimated from diagnosis date using the Kaplan-Meier estimator. Results: 747 patients with possible CD were identified, of whom 453 were confirmed to have CD by abstraction (172 UCD, 100 iMCD, 36 HHV-8 MCD, and 145 unclassified). IMCD patients were predominantly female (53%), white (58%), and treated at community sites (70%). Of the 52 iMCD patients with evidence of structured data within 90 days after diagnosis and who had at least one documented line of therapy, the most common first-line therapies were siltuximab-based therapy (42.3%), rituximab monotherapy (36.5%), and chemotherapy-based treatment (13.5%). Among 28 iMCD patients with evidence of second-line therapy, the most common treatment was rituximab monotherapy (35.7%). Among 60 iMCD patients diagnosed on or after siltuximab approval in April 2014 (including those without evidence of any treatment), 26 (43%) received siltuximab at some point. 5-year rwOS rate for the 100 iMCD patients was 75% [95% CI: 63-89%]. Conclusions: This is the first study to utilize a large EHR-derived database to describe characteristics, treatment patterns, and overall survival of iMCD patients in real-world practice. Less than half of iMCD patients diagnosed on or after the date of FDA approval for siltuximab received it at some point. Future work should focus on characterizing the drivers of poor patient outcomes.