Histoprognostic grade is a determinant parameter to select the initial therapeutic strategy in breast cancer (BC). Our aim was to analyse the grade repartition in BRCA1-BC and to explore the connections between grade and the BRCA1 gene function. We compared 27 BRCA1-BC from 14 families with 4461 cases from a registry and 242 sporadic cases, matching for grade, and constitutive elements, and then considered their repartition in families. We observed a prevalence of Grade 3 (p < 0.0001) in BRCA1-BC. This was attributed to nuclear polymorphism (p < 0.0001), mitotic activity (p < 0.0001) and to tubular differentiation (p = 0.0004), implying thatBRCA1 -BC are highly proliferating tumors. Moreover it is suggested that grade segregates as a genetic trait within families (p = 0.0015), and this was attributed to the mitotic index only (p = 0.0005). Thus grade, through its components, could be interpreted as the morphological translation of the BRCA1 germ-line mutation. Genotype-phenotype correlation may exist between the type of mutation and the aggressiveness of the disease. Such findings are bound to have important impact in the care management of hereditary breast cancer.