Extreme thrombocytosis in patients with ankylosing spondylitis (AS) is rarely reported. Because the relationship between high disease activity and increased platelet counts is somewhat contradictory, severe thrombocytosis in AS patients can be secondary to infection, iron deficiency anemia, drug administration, and hematologic malignancies. Essential thrombocythemia (ET) is a rare acquired stem cell neoplasm characterized by overproduction of platelets by megakaryocytes in the bone marrow in the absence of other causes of thrombocytosis. There is no report in the literature regarding the association between AS and ET. We report on a case of a 34-year-old Korean man with active AS diagnosed as JAK2V617F mutation negative ET during adalimumab treatment. (J Rheum Dis 2015;22:51-55)

Keywords: Ankylosing spondylitis, Essential thrombocythemia, Tumor necrosis factor alpha