Multicentric Castleman disease (MCD) is a rare lymphoproliferative disorder characterized by systemic lymphadenopathy and inflammatory symptoms that are associated with the overproduction of interleukin 6 (IL-6). Although several nonlymphoid organs can also be damaged in MCD, only a few cases with cardiac complications have been reported to date. We report a case of congestive heart failure in a female patient with MCD. On admission, her echocardiogram revealed a dilated and diffusely hypokinetic left ventricle. No stenosis was evident in the coronary angiogram. A histopathologic examination of a myocardial biopsy specimen showed mildly hypertrophic myocytes without infiltration of plasma cells or amyloid deposits. Repeated administration of an anti-IL-6 receptor antibody, tocilizumab (formerly known as MRA), gradually improved the ventricular wall motion over 6 months without any additional treatment for heart failure, suggesting the involvement of IL-6 in the pathogenesis of her cardiomyopathy. This report is the first of MCD complicated by heart failure treated successfully with tocilizumab. Administering tocilizumab in cases of MCD with unexplained cardiac dysfunction is worthwhile, because such a complication could be reversible.