Brain magnetic resonance imaging revealed leptomeningeal enhancement between the cerebellar folia (Figure 1A). Cerebrospinal fluid tests revealed white blood cell count of 16/µL (96% lymphocytes) (to convert white blood cell count to ×109 per liter, multiply by 0.001), red blood cell count of less than 1 ×106/µL (to convert to ×1012 per liter, multiply by 1.0), and normal protein and glucose levels; viral, fungal, and bacterial culture results were negative (including varicella-zoster virus, herpes simplex virus types 1 and 2, enterovirus, Lyme, and cryptococcus); and oligoclonal bands, cytology, and flow cytometry were unremarkable. Cerebrospinal fluid test results were negative for autoantibodies to antineuronal nuclear antibody 1 (anti-Hu), antineuronal nuclear antibody 2 (anti-Ri), antineuronal nuclear antibody 3, antiglial nuclear antibody 1, Purkinje cell cytoplasmic antibody type 1 (anti-Yo), Purkinje cell cytoplasmic antibody type 2, amphiphysin, and collapsin response mediator protein 5 (anti-CV2). Serum paraneoplastic panel results were negative, which included the same cerebrospinal fluid antibodies plus anti-Ma1, anti-Zic4, anti-GAD65, Purkinje cell cytoplasmic antibody type Tr, P/Q- and N-type calcium channel antibodies, voltage-gated potassium channel, N-methyl-D-aspartate receptor, and acetylcholine receptor antibodies. Serum studies revealed only an isolated elevated interleukin 6 level at 26 pg/mL (normal, <5 pg/mL). Results from human immunodeficiency virus RNA polymerase chain reaction and human herpesvirus 8 IgG and DNA testing were negative. Computed tomographic scans of the chest, abdomen, and pelvis found multiple anterior mediastinal soft-tissue nodules that were metabolically active on positron emission tomography with fluorodeoxyglucose. Thymic and lymph node biopsies confirmed the diagnosis of hyaline vascular Castleman disease (Figure 2).