This study was conducted to determine the prognostic influence of obvious peritumoral vascular emboli as prospectively determined by a simple routine slide examination in patients with operable node-negative breast cancer. Obvious peritumoral emboli (OPE) were defined by the presence of neoplastic emboli within unequivocal vascular lumina (including both lymphatic spaces and blood capillaries) in areas adjacent to but outside the margins of the carcinoma. OPE were assessed routinely on 5 microns thick haematoxylin and eosin-stained sections for each of 1320 primary operable node-negative breast cancers from 1975 to 1992 at our institution. OPE and other prognostic variables (tumour size, SBR grade, oestrogen and progesterone receptor status) were correlated to overall survival (OS) and metastasis-free interval (MFI) by means of univariate and multivariate analysis with a median follow-up of 103 months. OPE were found in 19.5% of tumours. In univariate analysis, OPE were related to tumour size (P = 6.3 x 10(-5)) and histologic grade (P = 4.9 x 10(-7)). Statistically significant correlations were found with OS (P = 4.6 x 10(-5)) and MFI (P = 6.4 x 10(-9)). Furthermore, in multivariate analysis, OPE was an independent prognostic variable, the most predictive factor for MFI (P = 7.7 x 10(-7)) before tumour size and grade, and was second after tumour grade for OS (P = 0.002). This study on a large unicentric series and with a long follow-up confirms the prognostic significance of vascular emboli in patients with operable node-negative breast carcinoma. Importantly, vascular emboli were found to be accurately detectable by a simple routine and non-time-consuming method. Therefore, such obvious vascular emboli should be considered as an important cost-effective, prognostic variable in patients with node-negative breast carcinoma.