The interobserver reproducibility of the Nottingham modification of the Bloom and Richardson histologic grading scheme for invasive breast carcinoma was tested. Six surgical pathologists from four institutions independently evaluated histologic grade and each of its three components for 75 infiltrating ductal carcinomas. The number of slides per case ranged from one to nine (median 3). Pairwise kappa values for agreement ranged from moderate to substantial (0.43-0.74) for histologic grade. Generalized kappa values indicated substantial agreement for tubule formation (0.64), moderate agreement for mitotic count (0.52), and near moderate agreement for nuclear pleomorphism (0.40). Normalizing the mitotic counts per mm2 showed only slight improvement in agreement over the published range of mitotic counts for three different field areas. The results suggest that steps to discriminate between categories for nuclear pleomorphism would likely be of benefit for improving the interobserver reproducibility of histologic grade. Nevertheless, the Nottingham modification of the Bloom and Richardson grading system is recommended as a suitable scheme for evaluating invasive breast carcinomas in the routine clinical setting.