Over a 21-year period, 213 women with mature teratoma of the ovary were operated on at Parkland Memorial Hospital. Twenty-seven had bilateral tumors. In 22 of these 27 patients, bilateral involvement was evident on clinical examination, and in 5, biopsy of an abnormal but nonteratoid appearing ovary was required to make the diagnosis. Ninety patients with a visually normal opposite ovary had no identifiable tumor in that ovary by investigative incision or incidental excision. Neither incision nor excision of the normal contralateral ovary was performed in 58 cases, and subsequently, one of these women is known to have developed a teratoma in that ovary. We conclude that synchronous covert bilaterality of mature teratomas is not common, and a visually normal contralateral ovary should not routinely be bivalved or wedge biopsied.