Immunoglobulin G4–related disease is an increasingly recognized, idiopathic systemic disorder that might be associated with elevated serum IgG4 level and tissue infiltration by IgG4-positive plasma cells. We describe the clinical features and biopsy findings in a patient who presented with features suggestive of pachymeningitis and multiple cranial neuropathies. Meningeal biopsy and other laboratory studies established the diagnosis of IgG4-related hypertrophic pachymeningitis. Despite treatment with corticosteroids and mycophenolate mofetil, the patient exhibited a fluctuating progressive course, which stabilized with rituximab, although the radiological findings persisted over 2½ years of follow-up. Our case highlights many important evolving concepts in the disorder, including unusual pathologic features, lack of correlation between serum IgG4 levels and the clinical course, and posttreatment clinicoradiological discordance. We provide potential explanations for this discrepancy, highlight the validity of novel cerebrospinal fluid studies and progressive systemic involvement despite use of immune-suppressive treatments, and emphasize the usefulness of rituximab as a disease-stabilizing agent.