Although many studies have found that ploidy as determined by flow cytometry correlates with grade for invasive mammary carcinomas, only a few groups of investigators have evaluated ploidy for infiltrating ductal carcinomas exclusively. In this study ploidy, as analyzed in fresh tumor specimens, was compared with grade (using the Bloom and Richardson grading scheme modified by Elston and Ellis) and each of its three components (tubule formation, nuclear pleomorphism, and mitotic rate) for 118 infiltrating ductal carcinomas. Two thirds of the neoplasms were DNA aneuploid, including 4% that were hypodiploid, 14% that were tetraploid, 8% that were hypertetraploid, and 12% that were multiploid. Ploidy correlated with nuclear pleomorphism (P = .004), mitotic rate (P = .001), and grade (P = .0007), but not with tubule formation (P = .09). Forty percent of grade I, 71% of grade II, and 83% of grade III neoplasms were DNA aneuploid. Ploidy was also compared with the results of combining nuclear pleomorphism with mitotic rate to form a five-part modified Bloom and Richardson scheme (MBRS grades I to V). The frequencies of DNA aneuploidy for tumors that were MBRS grades I, II, III, IV, and V were 25%, 65%, 66%, 73%, and 89%. This relationship was significant (P = .0006). When MBRS grades II, III, and IV neoplasms were combined and compared with MBRS grade I tumors and MBRS grade V carcinomas, the differences in frequency of DNA aneuploidy among these three groups also were significant (P = .00007). These results show that ploidy correlates with the grade of infiltrating ductal carcinoma and that nuclear pleomorphism and mitotic rate are more important than tubule formation for predicting the likelihood of DNA aneuploidy.