A 71-year-old woman presented with cutaneous ulcerations on her legs. The patient was not taking any medications and had no history of infections. A biopsy of the skin lesions revealed small-vessel neutrophilic vasculitis, and direct immunofluorescence revealed vascular IgM deposits. Serum protein electrophoresis revealed IgM κ monoclonal gammopathy (480 mg/dL; to convert to milligrams per liter, multiply by 10) and type I cryoglobulinemia (cryocrit concentration, 50%). The serum IgG level was normal (834 mg/dL [reference range, 700-1600 mg/dL]; to convert to grams per liter, multiply by 0.01). An exhaustive diagnostic workup revealed no underlying hematologic disease and no viral infection; a test for autoantibodies had negative results. A diagnosis of cutaneous necrotizing vasculitis associated with type I essential cryoglobulinemia was made. Treatment with oral prednisone (60 mg/d), intravenous pulses of methylprednisolone (500 mg/d for 3 days), and azathioprine sodium (150 mg/d for 3 months) was unsuccessful. Because of uncontrolled vasculitis, off-label therapy with a single cycle of 4 weekly infusions of rituximab at a dose of 375 mg/m2 was administered, azathioprine therapy was discontinued, and the prednisone dose was tapered to 50 mg/d, followed by a gradual tapering of 5 mg/mo. This regimen resulted in complete healing of the skin ulcers within 8 weeks. Four months after the completion of rituximab therapy, the patient presented with speech disturbance, behavioral changes, and weight loss. Brain magnetic resonance imaging showed multiple ring-enhancing lesions (Figure). Cerebral toxoplasmosis was suspected and confirmed by means of a polymerase chain reaction test for Toxoplasma gondii DNA in the cerebrospinal fluid and the identification of bradyzoites in the brain tissue. The patient was seronegative for human immunodeficiency virus (HIV) and had a normal neutrophil count (4380/μL), normal CD4 and CD8 lymphocyte counts (434/μL and 376/μL, respectively), B-lymphocyte depletion (1 cell/μL), and a low serum IgG level (1.90 g/L [reference range, 7-16 g/L]). The patient tested positive for Toxoplasma -specific IgG but negative for IgM, which suggested a reactivation of the disease from a prior infection. Despite treatment with pyrimethamine, sulfadiazine sodium, and folinic acid, the patient's mental status did not improve.