Several rare disorders characterized by histiocytic hyperplasia are inherited as single-gene Mendelian traits. These familial forms of histiocytosis are heterogeneous and can be clearly distinguished from each other by clinical and genetic criteria. The genetic nature of these disorders can lead to familial recurrence and thus necessitates their distinction from the classical reticuloendothelioses. The clinical manifestations and modes of inheritance of the various familial histiocytoses are reviewed.