Cell-mediated immune response was measured in 23 patients with ovarian cystadenocarcinoma, 38 patients with benign ovarian tumor, and 44 healthy volunteers. The method used two indexes: the lymphocyte response per unit volume of peripheral blood to phytohemaggluttnin (PHA) and the immunosuppressive effect of serum on the response of normal lymphocytes to PHA stimulation. The lymphocyte response per 50 μl peripheral blood did not differ significantly between patients with ovarian cancer and healthy volunteers. The serum effect, in contrast, differed significantly between malignant and benign ovarian tumors, and was found to increase significantly even when the cancer masses were as small as about 5 X 5 X 5 cm in size, ie, in FIGO Stage 1. It is our belief that the measurement of the serum effect in patients with any ovarian tumor enables the early detection of ovarian cancer.