Reactive AA amyloidosis is caused by the accumulation of the acute phase reactant, serum amyloid A (SAA), as a complication of chronic inflammatory conditions. Cyclic neutropenia is a rare hereditary disorder characterized by repeated episodes of neutropenia at regular intervals, with or without concurrent infection, and is known to be a rare cause of AA amyloidosis. Here, we report a case of a patient who developed systemic AA amyloidosis following a prolonged course of undiagnosed cyclic neutropenia. The patient had a history of recurrent infections since infancy and developed goiter, proteinuria, and azotemia at age 14 years. Her SAA level was markedly increased (601.8 μg/mL, normal range <8 μg/mL), and a thyroid and kidney biopsy revealed typical lesions of AA amyloidosis. Amyloid deposits were also detected in the myocardium, colon, and gallbladder. She had repeated episodes of neutropenia regularly at 3-week intervals and a pathogenic mutation in the ELA2 gene. After 10 months of treatment with recombinant human granulocyte colony-stimulating factor, her SAA level normalized (<2.5 μg/mL), but her renal function did not recover. This case clearly shows that cyclic neutropenia can be complicated by AA amyloidosis unless it is detected early and treated adequately.