Objective: The purpose of this study was to determine the incidence and the mammographic appearance of polyhedral microcalcifications., Materials and Methods: Prospectively, we evaluated screening mammograms in 2000 women for polyhedral microcalcifications. The number of polyhedral microcalcifications on routine and magnification views was established, and a quantitative analysis was done to determine if the shape of the polyhedral microcalcifications varied in the different projections. The ratio between the longer axis (R) and the shorter axis (r) was calculated., Results: Two radiologists detected at least two polyhedral microcalcifications on one projection in 58 (3%) women. The number of polyhedral microcalcifications detected ranged from 2 to 47 (mean, 8.2) for screening mammograms and from 2 to 62 (mean, 13.4) on magnification views. Polyhedral microcalcifications were bilateral in 22 cases, scattered in one breast in 19 cases, segmental in 10 cases, and grouped in a cluster in seven cases. Lateral projections showed more rhombohedral microcalcifications, and craniocaudal projections showed more square microcalcifications., Conclusion: The frequency of polyhedral microcalcifications is 3%. The shape of polyhedral microcalcifications varies: craniocaudal views show them to be square and lateral views show them to be rhombohedral.