Stanniocalcin (STC) is a glycoprotein hormone that regulates calcium levels in fish. The related human protein stanniocalcin-1 (STC-1) is expressed in multiple organs including the ovary, prostate, kidney, and thyroid. Human STC-1 expression is enhanced by neural cell differentiation, bone formation, and cell proliferation. Recently, quantitative RT-PCR analysis showed high levels of STC-1 mRNA in the blood of patients with cancer as compared with those in the blood of volunteers without cancer. The STC-1 mRNA expression in blood is closely related to tumor size in breast cancer, micrometastases of hepatocellular carcinoma, and minimal residual disease in leukemia. This brief review describes the diagnostic significance of STC-1 as a molecular marker for human cancer.