Matrix metalloproteinases are belived to play a central role in the invasion and metastasis of human cancers by mediating the degration of extracellular matrix components. Expression of stromelysin-3 (ST3), a new member of matrix metalloproteinase family, was investigated by in situ hybridization in 32 tissue samples of medullary carcinoma of the breast. Eighty-four per cent of the cases showed ST3 gene expression in stromal cells adjacent to tumor cells. Therefore, expression of ST3 in stromal cells may be expected to play a significant role in the destruction of extracellular matrix and invasion of medullary carcinoma of the breast.