Records of 1,400 patients with the diagnosis of venous thrombosis or thrombophlebitis were reviewed. Eighty-one had associated cancer. In 61, the thrombophlebitis was diagnosed clinically; in 20, at autopsy. Fifty-one had carcinoma. Thrombophlebitis was recognized in 31 prior to the carcinoma. Other types of malignancy also showed this relationship. The lungs, female reproductive tract, and the pancreas were most frequent sites for the primary lesion. The resistance to anticoagulant therapy used in the treatment of thrombophlebitis was striking. Phlebitis sometimes persists or recurs over long periods of time before the cancer is discovered. These characteristics should be regarded as warning signs indicating a comprehensive survey for occult cancer.