We report a 44-year-old woman who had anti-aquaporin 4 (AQP4) antibody-positive myelitis and immune thrombocytopenic purpura (ITP). She was admitted to our hospital with paraparesis, dysesthesia below the Th8 dermatome level on her right side and lower extremities, constipation and urinary retention. Magnetic resonance imaging revealed a longitudinally extending lesion at the level of Th4-Th10. Her serum sample was positive for anti-AQP4 antibody. Corticosteroid therapy was initiated, and her symptoms were largely ameliorated. Furthermore, concurrently with the myelitis, her platelet count dropped (99 × 10(9)/l). A diagnosis of ITP was made with positive serum platelet-associated IgG (PA-IgG) and negative work-up for blood malignancies by bone marrow aspiration. Since a causal relationship between Helicobacter pylori (H. pylori) and ITP is suggested by several studies, she was also examined and diagnosed with H. pylori-positive ITP. After the bacteria eradication therapy, her platelet count and PA-IgG returned to normal range. Furthermore, the anti-AQP4 antibody titer declined and her symptoms were almost resolved. We considered that H. pylori might influence progression of the myelitis as well as induction and development of ITP.