Castleman disease (CD) is a very rare, non-malignant lymphoproliferative disorder that can be classified as unicentric or multicentric (MCD). MCD is associated with systemic symptoms, including organ dysfunction due to cytokine dysregulation, primarily interleukin-6 (IL-6). The anti-IL-6 monoclonal antibody siltuximab is recommended as a frontline treatment for idiopathic MCD (iMCD), but real-world data on its use in routine clinical practice are limited. This study aimed to assess disease response and survival outcomes in patients with iMCD treated with siltuximab therapy in real-world settings in Greece and Romania.This retrospective cohort study included adult patients with iMCD treated with siltuximab in clinical practice across Greece and Romania between January 2017 and December 2022. The primary endpoint was overall response rate and secondary endpoints included survival and safety outcomes. Response assessments were performed according to the Castleman Disease Collaborative Network guidelines. Patients were followed until death, loss to follow-up or study conclusion (October 2023).Forty-eight patients with iMCD were included in the study. Mean age at baseline was 65 years, with significant age differences between patients from Greece (74 years) and Romania (54 years). The majority of patients were male (68.8%) and received one prior line of therapy (75%). Patients included in the study received a median of nine cycles of siltuximab. Response data were available for 38 patients. The overall response to siltuximab was 71.1%, with 55.3% of patients achieving a complete response, and 15.8% a partial response. The estimated overall survival rate at 3 years was 74% and the median survival was 123 months. The most common adverse events (> 5%) included elevated liver enzymes, anxiety, allergic reactions and nausea/diarrhea. Serious adverse events were experienced by 16.7% of the patients.Our results suggest that siltuximab-based therapy is effective in treating iMCD in real-world settings in Greece and Romania. To our knowledge, this study represents the largest real-world analysis of siltuximab in European patients with iMCD so far.