Approximately 20% of patients with invasive mammary carcinoma who do not have axillary metastases develop recurrent carcinoma within 10 years of initial therapy. There is clearly a need to identify those patients most likely to develop recurrences in this group since they may benefit from adjuvant therapy. This study was undertaken to evaluate the prognostic significance of intramammary lymphatic tumror emboli in patients with invasive breast carcinoma who did not have lymph node metastases. Twenty-three such patients treated in 1974 and 15 from 1964 were compared with matched groups of control patients who did not have lymphatic emboli. About 43% of patients with lymphatic emboli and 4% of those without emboli followed for 5 or more years in the 1964 group developed distant metastases (p less than 0.001). Local recurrences were found in only one study patient and one control in the entire series of 1964 and 1974 patients. The results suggest that among patients without axillary metastases, the finding of tumor cells in lymphatic spaces within the breast is associated with a substantial risk of distant metastases but not local recurrence.