The invasion of vascular spaces (lymphatic and/or blood vessel) by tumour, as assessed on routine haematoxylin and eosin sections, was investigated in a consecutive series of 1704 women with primary operable invasive breast carcinoma. Strict morphological criteria were used. Patients were under 70 years of age and received definitive surgery with no adjuvant systemic therapies. Information from regular follow-up (range 3-17 years) was recorded on to a computer database. Definite vascular invasion was seen in 22.8% of cases and concurrence between pathologists was high. In univariate analyses, vascular invasion was strongly associated with lymph node stage (P < 0.0001), tumour size (P < 0.0001), histological grade (P < 0.0001) and type of tumour (P < 0.0001). In multivariate analyses vascular invasion was of independent prognostic significance for both survival and for local recurrence of tumour; patients with tumours showing no vascular invasion had a significant survival advantage and a reduced risk of local recurrence. No association with oestrogen receptor status or menopause status was seen. The results confirm that histological assessment of vascular invasion provides independent prognostic information in primary operable breast carcinoma which may be helpful in making clinical decisions.