BACKGROUND: HIV-associated multicentric Castleman disease is a rare lymphoproliferative disorder with marked systemic symptoms attributed to cytokine disarray. Many therapeutic approaches in small series of patients have proved largely unsuccessful to date. OBJECTIVE: To investigate the efficacy and clinicopathologic variables associated with first-line treatment for HIV-associated multicentric Castleman disease with the anti-CD20 monoclonal antibody rituximab. DESIGN: Single-group, open-label, phase II trial. SETTING: 3 teaching hospitals in England. PATIENTS: Previously untreated patients with histologically proven HIV-associated multicentric Castleman disease. INTERVENTION: 4 infusions of rituximab, 375 mg per m2 of body surface area, at weekly intervals. MEASUREMENTS: Response was evaluated clinically and radiologically and by measuring plasma Kaposi sarcoma-associated herpesvirus viral load. RESULTS: 21 consecutive patients (18 men) with plasmablastic multicentric Castleman disease were recruited. The median follow-up was 12 months (range, 1 to 49 months). One patient died before completing therapy, 20 achieved remission of symptoms, and 14 (67%) achieved a radiologic response. The overall and disease-free survival rates at 2 years were 95% (95% CI, 86% to 100%) and 79% (CI, 49% to 100%), respectively. Plasma acute-phase proteins, immunoglobulins, and Kaposi sarcoma-associated herpesvirus viral load decreased after rituximab therapy. The main adverse effect was reactivation of Kaposi sarcoma. LIMITATION: The study had no comparison group. CONCLUSION: Rituximab may be clinically valuable as initial therapy for HIV-associated multicentric Castleman disease.