Insulin-like growth factor (IGF)-binding proteins (IGFBPs) modulate the actions of IGF. We have previously reported that IGFBP-2 messenger ribonucleic acid (mRNA) and protein are increased, and IGFBP-3 protein decreased in malignant prostate epithelium compared to benign epithelium. In this study, we examined the other IGFBPs secreted by prostate cells in vitro, namely IGFBP-4, -5, and 6. Immunoreactivity and mRNA signals for IGFBP-4 and -6 were localized to epithelial cells, with less signal in stroma. IGFBP-4 immunostaining and hybridization signal were significantly increased in prostate adenocarcinoma compared to those in benign epithelium. Immunostaining for IGFBP-5 was localized to the epithelium and stroma. IGFBP- 5 immunoreactivity was significantly increased in malignant compared to benign epithelium. IGFBP-5 mRNA signal was not localized to epithelial cells; rather, the signal was over stromal cells surrounding the acinar structures. These cells are thought to be fibroblasts. We show that IGFBP-4 mRNA and protein and IGFBP-5 protein are increased in malignant epithelium compared to benign epithelium, that IGFBP-6 is present in benign and malignant epithelium, and that there is differential localization of IGFBP-5 mRNA and protein in prostate tissue. IGFBP-5 that is made by fibroblasts appears to be sequestered by epithelial cells. IGFBP-5 may, therefore, be a factor in cellular interactions between stromal and epithelial cells that are of fundamental importance for normal prostatic development and function.