The case of a 21-year-old woman with findings consistent with Costello syndrome (CS) is reported. Most previously reported cases of CS were in young children. In addition to the classic features, the patient had endocrine disturbances including secondary amenorrhea, goiter, and adrenal insufficiency. This last abnormality may be the cause of the hypoglycemic episodes seen in CS. Osteoporosis and anemia were additional features. This case provides new information on the endocrine features and prognosis of CS.