Recently there has been an increase in the detection of DCIS, and there is much interest in establishing a new pathologic classification that has less subjective criteria than traditional methods and that more closely reflects the clinical behavior of the tumor. It is hoped that this will lead to a more rational approach to treatment of DCIS. In this study one such recently proposed classification has been applied to the DCIS component of 215 infiltrating ductal carcinomas, and this has been compared with the histologic grade of the tumors. There was a highly significant correlation between DCIS type and grade (chi 2 = 110.0, P < 0.0001); well- and intermediately differentiated DCIS were usually found in grade I and grade II tumors, respectively. Poorly differentiated DCIS was equally associated with grade II and grade III infiltrating carcinomas. Furthermore, in univariate analysis, the DCIS type correlated with disease-free (chi 2 = 17.1, P < 0.001) and overall survival (chi 2 = 13.97, P < 0.001). In a Cox multivariate analysis, which included histologic grade, the association between DCIS type and prognosis was lost because of the very close relationship between type and grade. These results support the validity of the new classification and reinforce the view that treatment of pure DCIS should be related to tumor type.