AIM: To determine the prognostic significance and clinical relevance of cathepsin D detected by immunocytochemical assays (ICAs) in breast carcinomas. METHODS: 151 patients presenting with palpable or impalpable breast carcinomas and who had not received any kind of adjuvant chemotherapy or endocrine therapy who were operated from January 1986 to May 1987 were studied. ICAs of tumour specimens were performed in optimal technical conditions (frozen sections, automated immunoperoxidase technique (Ventana), and computer assisted analysis of digitised coloured microscopic images (SAMBA)) to determine cathepsin D immunocytochemical expression. Results of quantitative ICAs were correlated with overall and disease free survival over 8.4 years of follow up in axillary lymph node positive and negative patients. RESULTS: Cathepsin D immunocytochemical expression in tumours of 15% or more was significantly associated with poor overall survival in the whole group and in node positive patients (Kaplan Meier, log rank test p = 0.003 and p = 0.007); however, it was not correlated with survival in node negative patients. Cathepsin D immunocytochemical expression (> 15%) correlated with short disease free (p = 0.015) and short recurrence free survival (p = 0.021) in the group as a whole but not when node positive and negative patients were evaluated separately. CONCLUSIONS: In optimal conditions (automated and quantitative ICAs on frozen sections) cathepsin D immunohistochemical expression is a significant prognostic indicator in terms of overall, disease free, and recurrence free survival; however, there is no correlation when node negative patients are considered separately.