We report the case of 19-year-old woman with cyclical Cushing's disease, in whom plasma adrenocorticotropin (ACTH) was secreted periodically after her first pregnancy. Since the 33rd week of pregnancy, hypertension and proteinuria became clinically remarkable. She gave normal birth at 36th week of pregnancy; however she continued to gain body weight even after delivery and developed typical Cushingoid features. Her ACTH secretion lacked normal daily fluctuation but exhibited periodic change during 1-year observation, showing 119 pg/ml, 34.6 pg/ml and 115 pg/ml at the 4th, 7th and 13th months after delivery. Plasma ACTH levels were increased by corticotropin releasing hormone and metyrapone, while low-dose dexamethasone suppressed cortisol secretion. Gel filtration analysis of the patient's plasma detected big ACTH molecules being eluted with a peak of authentic 1-39 ACTH. Cranial magnetic resonance imaging revealed a 1-cm pituitary mass in right cavernous sinus. The pituitary tumor was removed by transsphenoidal surgery at 13th month after delivery and was pathologically compatible with ACTH-producing pituitary adenoma by immunohistochemistry. This case includes clinically rare subsets of Cushing's syndrome showing periodic ACTH secretion and aberrant ACTH molecules.