Estrogen is known to increase exogenous corticosteroid levels. In this case, a 27-year-old Japanese woman was referred to our hospital for examination of an adrenal tumor and was diagnosed with Cushing syndrome. Resection of the tumor resulted in secondary adrenal insufficiency. She also developed microcytic anemia due to hypermenorrhea, which was masked by Cushing syndrome. An oral contraceptive was administered for the treatment of hypermenorrhea, but this led to a marked increase in serum cortisol and the reduction of plasma adenocorticotropic hormone, disturbing the recovery of the adrenal function. Attention is required when oral contraceptives are used to treat hypermenorrhea masked by Cushing syndrome.