Ataxia-telangiectasia, an inherited disorder characterized by progressive cerebellar ataxia and telangiectasias, is often associated with primary immunodeficiency and high incidence of malignancies, mostly of the lymphoreticular type. Endodermal sinus tumor is a rare germ cell tumor of the ovary characterized by an extremely rapid growth and poor prognosis. Both these diseases are associated with an abnormal production of alpha-fetoprotein. Primary tumors of the ovary in patients with ataxia-telangiectasia are extremely rare and the association of an endodermal sinus tumor and ataxia-telangiectasia has never been reported in the literature. This case report serves to focus on the particular problems encountered in the diagnosis and management of two diseases both characterized by the same serum marker.