Cyclic neutropenia is a rare disorder characterized by repetitive episodes of neutropenia every 14 to 28 days. During the neutropenic periods, patients may have fever, recurrent aphthous stomatitis, and occasionally infections. Usually, the disease begins in childhood and a genetic mutation in the elastase gene (ELANE) has been reported. However, adult-onset cyclic neutropenia is considered an autoimmune disorder in association with proliferation of cytotoxic T cells. We report here a case of 22-year old man with adult-onset cyclic neutropenia presenting with fever and polyarthritis. He had periodic neutropenia with intervals of about 21 days and his arthritis symptoms were aggravated simultaneously with the neutropenic episodes. Additionally, he had oral ulcer, abdominal pain, or fever in the neutropenic period. Bone marrow study, peripheral blood smear, peripheral lymphocyte subset analysis, and ELANE sequencing did not show any abnormal findings. Initial trial of cyclosporine had no effect on his cyclic neutropenia episodes. After regular administration of G-CSF subcutaneous injection, the duration of neutropenia was shortened and his arthritis was improved. To the best of our knowledge, this is the first case of adult-onset cyclic neutropenia in Korea.