There are no suitable histologic diagnostic clues for determining the true biological malignancy of invasive ductal carcinomas associated with lymph vessel tumor emboli. The purpose of this study was to devise a grading system for lymph vessel tumor emboli in invasive ductal carcinomas that would allow accurate prediction of the outcome of invasive ductal carcinoma patients with lymph vessel invasion. We classified 393 invasive ductal carcinomas into the following 4 grades according to the number of mitotic and apoptotic figures in tumor cells in lymph vessels at 1 high-power field: grade 0, no lymph vessel invasion; grade 1, absence of mitotic and apoptotic figures, presence of any number of mitotic figures and absence of apoptotic figures, or absence of mitotic figures and presence of any number of apoptotic figures; grade 2, 1 to 4 mitotic figures and 1 or more of apoptotic figures, or 1 or more of mitotic figures and 1 to 6 apoptotic figures; and grade 3, more than 4 mitotic figures and more than 6 apoptotic figures. The mortality rate increased with the grade, and the mortality rate of patients with grade 3 lymph vessel tumor emboli was more than 70%. Multivariate analyses with well-known prognostic factors demonstrated that grade 3 lymph vessel tumor emboli significantly increased the hazard rates for tumor recurrence, and tumor death independent of adjuvant therapy status, nodal status, or invasive tumor size. The grading system for lymph vessel tumor emboli is the best histologic grading system for accurately predicting the outcome of patients with invasive ductal carcinoma of the breast.