BACKGROUND Human kallikrein 10 (hK10, encoded by KLK10 gene) is a recently discovered member of the human kallikrein family. hK10 is a secreted serine protease. With the development of a highly sensitive and specific immunoassay for hK10, quantification of hK10 in the circulation is now feasible. Our aim was to investigate whether hK10 concentration in serum changes in various malignancies. METHODS We used a highly specific and sensitive immunofluorometric assay to quantify hK10 protein in 374 serum samples from healthy individuals and patients with various malignancies. RESULTS Serum hK10 concentration was found to be significantly elevated in 56% of the ovarian cancer patients and such an increase was not observed in serum of healthy individuals or in serum of patients with other types of cancer, with the exception of approximately 15% of patients with gastrointestinal cancer. This hK10 elevation does not correlate well with CA 125. We have further demonstrated that hK10 concentration changes during ovarian cancer progression. CONCLUSION This is the first report describing that hK10 serum concentration is significantly elevated in the majority of ovarian cancer patients. Our results indicate that hK10 may be a potential new serological marker for ovarian cancer diagnosis and monitoring.