BACKGROUND: Many members of the human kallikrein gene family were found to be differentially expressed in various malignancies and some of them are useful diagnostic/prognostic markers. KLK9 is a newly discovered human kallikrein gene that is expressed in several tissues including thymus, spinal cord, testis, prostate, breast, and ovary. Like other kallikreins, the KLK9 gene was found to be regulated by steroid hormones, mainly estrogens and progestins, in cancer cell lines. EXPERIMENTAL DESIGN: We studied the expression of KLK9 by quantitative RT-PCR in 169 breast cancer patients of different stages, grades and histological types. We also compared the relation between KLK9 expression and other clinicopathological variables and patient survival. RESULTS: KLK9 expression is significantly higher in patients with early stage cancers (p = 0.039) and in patients with small tumor size (