Thirty cases of ductal carcinoma in situ (DCIS) of the breast, showing histological microcalcifications, were studied to clarify their mechanism of formation. Undecalcified sections revealed three types of calcium precipitates: type I and II granular calcifications (GCs) and laminar calcifications (LCs). In type I GCs the core on which the calcium had deposited was constituted mainly by nuclear debris. Type II GCs were predominantly composed by mucosubstances. LCs were the result of calcium deposits on mucoid or proteinaceous material, arranged in concentric lamellae. LCs and type II GCs were mainly present in well and intermediately differentiated DCIS. Type I GCs were observed in only DCIS with necrosis, frequently being present in intermediately and poorly differentiated DCIS.