Patients with dysgenetic gonads and Turner syndrome are unlikely to develop endometrial carcinoma unless they have received unopposed estrogen replacement therapy. This case describes a 54-year-old woman with Turner syndrome and primary amenorrhea who developed adenocarcinoma of the endometrium without having received hormone replacement. Vaginal bleeding, a pelvic mass, and sepsis were the presenting symptoms. The patient also had diabetes mellitus and hypothyroidism. Polyglandular endocrine patterns are known to occur with a high frequency in these patients. The woman's chromosome studies revealed a modified 46,X,i(Xq) (isochromosome X). This is the first report of an isochromosome X patient to develop endometrial cancer without receiving estrogen replacement. The etiology of this rare case may be an increased propensity for patients with X-chromosome deletions to develop neoplasms in general, or extragonadal estrogen production.