The immunohistochemical expression of the aspartyl protease enzyme cathepsin D was examined in a consecutive series of 103 primary operable breast carcinomas with the polyclonal antibody NCL-CDp. Expression of cathepsin D was identified within the epithelial and stromal components of all tumours examined. No significant associations of increased cathepsin D expression in the epithelial tumour component with conventional prognostic indices such as tumour size, grade, lymph node stage, or patient survival were identified. However, significant associations of increased stromal cathepsin D expression and high tumour grade, chi 2 = 11.40 (df = 2), p = 0.003; increased tendency to local recurrence, chi 2 = 6.87 (df = 1), p = 0.009; regional recurrence, chi 2 = 7.44 (df = 1), p = 0.006; poorer disease free survival, chi 2 = 14.9 (df = 1), p = 0.0001; and poorer overall patient survival, chi 2 = 6.90 (df = 1), p = 0.0086, were identified. Cathepsin D expression is present in all breast tumours. Stromal cathepsin D expression is a neglected immunohistochemical prognostic parameter which could explain some of the previous apparently conflicting reports concerning the effect on patient prognosis of biochemical (i.e. total) and immunohistochemical estimations of cathepsin D in breast cancers.