The expression level of stromelysin-3 (ST3) mRNA was analyzed by in situ hybridization of formalin-fixed, paraffin-embedded primary breast-tumor samples from 76 patients. Digital image analysis of the dark-field in situ hybridization signal was used to measure the maximal level of ST3 expression in each tumor. All 55 invasive ductal carcinomas and 9 of 10 invasive lobular carcinomas were positive for ST3. Invasive tumors had significantly higher levels of ST3 than in situ tumors. Furthermore, ST3 levels were higher in invasive ductal carcinomas than in invasive lobular carcinomas. The ST3 expression level was significantly correlated to fatal metastatic disease (mean follow-up 104 months). ST3 levels of < 2,500 units were associated with distant metastasis in 46% of patients, whereas levels of > 2,500 units were associated with metastasis in 79% of patients selected for study. ST3 mRNA levels did not correlate with tumor size, microvessel density, DNA ploidy or estrogen-receptor levels. Studies of ST3 expression may provide information valuable for the understanding of breast cancer biology and for prognosis.