Expression of RCAS1 (receptor-binding cancer antigen expressed on SiSo cells) is associated with advanced disease of various malignancies including ovarian cancer. Proteolytic cleavage of RCAS1 at extracellular domains (ectodomain shedding) yields soluble RCAS1. Although RCAS1 can induce apoptosis in normal peripheral lymphocytes, its clinical significance and biologic function in ovarian cancer patients are unclear. Here, we evaluated serum RCAS1 concentrations to clarify its clinical significance and biologic activity in ovarian cancer. Via ELISA, we measured serum RCAS1 concentrations in samples from 75 healthy blood donors and 97 patients, 36 with ovarian benign tumor and 61 with ovarian cancer. We correlated via statistical means the RCAS1 values with patients' clinicopathologic variables. We assessed inhibition of growth of K562 cells, which express the putative RCAS1 receptor, via WST-1 assay of serum samples to clarify RCAS1's biologic activity. Ovarian cancer patients had significantly higher serum RCAS1 concentrations than did healthy blood donors and ovarian tumor patients (P<0.05). RCAS1 level was significantly different according to histologic subtype for both ovarian tumor and cancer patients (P=0.0266 and 0.0074, respectively). RCAS1 values were also significantly associated with response to treatment (P<0.001). The WST-1 assay showed that patients' serum induced K562 cell growth inhibition, but this effect partially recovered after immunodepletion of RCAS1 (P=0.0074). RCAS1 may be a biomarker of ovarian cancer by virtue of its ability to predict results of treatment and inhibit immune cell growth.