A 59-year-old white female presented with an acute polyarthritis, fever, and mental confusion. Accompanying blood changes included positive tests for rheumatoid factor and antinuclear factor, anemia, leukocytosis, thrombocytosis, and a circulating factor that stimulated serotonin release from normal platelets. Exploratory laporatomy for a mobile pelvic mass revealed an adenocarcinoma of the left ovary which was completely resected. Shortly thereafter the patient's acute symptoms subsided. After 2 years of follow-up the patient has remained free of symptoms and the serologic changes have reverted to normal.There is a general awareness that a polyarthritis resembling rheumatoid arthritis can be the initial manifestation of malignancy (1-4). The paucity of reports on this complication of cancer, early recognition of which can save a patient's life, prompted this detailed report of From the Section