RESUMEN
INTRODUCTION: A high platelet count, or thrombocytosis, is either a reactive process or a result of a myeloproliferative disorder. Ankylosing spondylitis is a chronic inflammatory rheumatic disease affecting the spine and sometimes peripheral joints in which reactive mild to moderate thrombocytosis is a common finding. There have been no previously reported cases of essential thrombocythemia associated with ankylosing spondylitis. CASE OUTLINE: We report a case of a 32-year-old man with human leukocyte antigen B27-positive ankylosing spondylitis and Janus kinase 2-positive essential thrombocythemia who was treated first with a combination of anagrelide and disease-modifying antirheumatic drugs and, after liver toxicity, with a combination of anagrelide and etanercept (TNF-α antagonist). Both diseases were gradually brought under control. CONCLUSION: Our case of ankylosing spondylitis and essential thrombocythemia suggests that concomitant etanercept and anagrelide therapy is safe, as well as effective.