Nineteen cases of pronounced eosinophilia associated with malignant tumors were reviewed from the literature. Fifteen additional cases, an incidence of 0.54% of 2363 cases of malignancy, were reported in this study. In all cases, repeated white blood cell counts confirmed the presence of an eosinophilia of 10% or higher. Other causes of eosinophilia were excluded. The occurrence of eosinophilia did not depend upon the type of malignancy. Proved metastases were present in 90% of the total of 34 cases; in 7% they were suspected but not proved. In 3% metastases were not suspected and not proved. Eosinophilia, when it occurs with malignant tumors, and when other causes can be ruled out, is indicative of dissemination, and consequently significant of a poor prognosis.