Topographic relationships to adjacent structures were used as criteria to identify intramammary lymphatics with tumor emboli in breast cancer patients, in addition to conventional morphologic criteria. Patterns of relationship to blood vessels, non-neoplastic lobules and ducts, and empty lymphatics were defined. Ninety-five cases were independently reviewed by two observers. Interobserver reproducibility of the diagnosis of lymphatic vessel invasion (LVI) was 82% (kappa 0.60). The observers agreed on the presence of LVI in 23 patients (24%), of whom 21 (91%) had positive lymph nodes. Only among patients in whom more than ten emboli were identified was the frequency of positive lymph nodes markedly higher than in the total material. The location of tumor emboli relative to the invasive tumor was of little significance. LVI was a more powerful predictor of lymph node status than tumor size, margin contour, histologic grade and histologic type, and was highly significant also when controlled for these features.