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Background: The aim of this study was to evaluate the pathological and morphological features of ductal carcinoma in situ (DCIS) within and surrounding invasive ductal carcinoma, and to investigate its relationship with clinical outcome and established prognostic variables. Methods: One hundred and seven patients with primary operable invasive breast carcinoma and associated DCIS treated by simple or subcutaneous mastectomy or wide local excision with radiotherapy were assessed. Those with pure DCIS and insufficient tumour available for examination were excluded. The most representative haematoxylin and eosin-stained sections from the remaining 91 samples were selected and examined at × 100 magnification using a 45-point, 2-mm grid graticule. The entire section was assessed and the cell under each point of the graticule was classed as either normal (a), DCIS surrounded by normal tissue (b), invasive tumour (c) or DCIS surrounded by invasive malignancy (d). The volume ratio of DCIS in the normal (b/(a + b)) and invasive (d/(c + d)) tissue was then calculated. Results: The DCIS volume within invasive tumour was not associated with outcome. The DCIS volume within adjacent normal tissue, however, was associated with local recurrence (P = 0·025), disease-free interval (P = 0·048), the occurrence of distant metastases (P = 0·019), death (P = 0·049) and disease-free survival (P = 0·048). Volume ratios of DCIS in normal and invasive tissue were not related to known prognostic factors including lymph node stage, grade, tumour size, vascular invasion or patient age. Conclusion: There is a significant prognostic effect relating to the extent of DCIS associated with an invasive cancer, particularly with respect to local recurrence of tumour. This effect is restricted to the volume of DCIS in the tissue surrounding the invasive lesion rather than the intratumoral component. [ABSTRACT FROM AUTHOR]