The effects of human primary prostatic stromal cells on the migration and morphogenesis of human prostatic epithelial cells, derived from tumor or benign prostatic hyperplasia tissue, were studied using a three-dimensional coculture system. Epithelial cells from tumor or benign tissue migrated efficiently into collagen gels populated with stromal cells from benign tissue. Only epithelial cells from benign prostate formed acinus-like structures that exhibited differentiated prostatic function and strong expression of membrane-associated E-cadherin. In gels populated by stromal cells from tumor tissue, migration of primary prostatic epithelial cells did not occur. In the absence of stromal cells, primary epithelial cells were unable to proliferate. This three-dimensional culture system allows closely controlled manipulation and analysis in vitro of interactions between prostatic epithelial and stromal cells.