Neuromyelitis optica (NMO) and NMO spectrum disorders (NMOSD) are associated with autoantibodies that target aquaporin-4 and, in many cases, multiple other autoantibodies, including antinuclear antibody and antibodies to extractable nuclear antigens. The clinical syndromes that define NMO and NMOSD, especially longitudinally extensive transverse myelitis and optic neuritis, can also occur in the context of established rheumatologic diseases such as systemic lupus erythematosus and Sjögren syndrome and other organ-specific autoimmune diseases. These observations raise questions fundamental to both clinical practice and etiologic research. For example, they could suggest that NMO is one manifestation of a genetic tendency toward humoral autoimmunity. Alternatively, they might indicate that NMO is a central nervous system complication of a multisystem rheumatologic disease. We describe the historical background of this controversy, summarize the current evidence that addresses NMO-systemic autoimmunity relationships, and discuss the practical implications for clinical management.