Background: Castleman Disease (CD) encompasses a diverse range of lymphoproliferative disorders, featuring subtypes like unicentric CD (UCD), multicentric CD (MCD), HHV-8 associated multicentric CD, and HHV-8 negative/idiopathic MCD (iMCD). HDN is a rare and aggressive lymphoma often linked with HHV8-positive multicentric Castleman disease (CD). It often manifests without any other obvious symptoms and may be confused with other causes of lymph nodal enlargement. The epidemiology, treatment patterns, and survival outcomes of HDN are poorly understood, prompting this study to explore CD's epidemiology using the National Cancer Institute's SEER database. Methods: Using SEER 2000-2020, we identified 77 CD cases through ICD Code 9738 for Large B Cell Lymph HHV8 Multicentric Castleman Disease. Demographic, socio-economic, clinicopathologic characteristics, and survival outcomes were assessed. Secondary-primary malignancy risk (SIR) and mortality risk (SMR) were both calculated with observed to expected (O:E) ratios from the general population, with significance set at P < 0.05 and 95% CIs. Results: The median age at diagnosis was 51.7 years. The primary site distribution was mainly nodal (77.9%), while the extra nodal sites were rarely involved (22.1%). Only 3.8% of patients received radiotherapy, while 50.6% received chemotherapy. The patients were predominantly male (80.5%) and White (63.6%). Patients with a household income above $65,000 accounted for 75.3%. At diagnosis, 33.7% of the patients were married. Survivors had increased risk of malignancies at all sites (O:E 7.75, 95% CI: 0.94-28.01) and significantly increased risk of all solid tumors (O:E 8.95, 95% CI: 1.08-32.32), malignancy of kidney and renal pelvis (O:E 88.37, 95% CI: 2.24-492.36) and Kaposi Sarcoma (O:E 2,329.89, 95% CI: 58.99-12,981.34) within one year. The two most common causes of death in the CD cohort were infectious and parasitic diseases, including HIV, and malignant cancers. The 1-year, 3-year and 5-year Overall survival was 63.6%, 59.7%, and 54.4% respectively, and the corresponding Cancer-specific survival was 68.8%, 68.8%, and 65.7%, respectively. Conclusions: This study sheds light on the epidemiology of HDN. Despite the rarity of HDN, our analysis of data reveals key demographic and clinical characteristics of HDN patients. The survival outcomes indicate a challenging landscape, with notable risks of secondary malignancies, particularly within the first-year post-diagnosis. The identification of specific risks, treatment patterns, and survival rates contributes valuable insights for future research and clinical management strategies in addressing this aggressive lymphoma.