The sea-blue histiocyte syndrome, similar to Niemann-Pick disease, is a congenital, hereditary histiolipidosis due to an inborn enzymatic error. Accumulation of non saturated, oxidated, polymerized lipids is observed; ceroids of lipofuscin, glycophospholipids and sphingomyelin, like bulky granules 1 to 3 u in diameter, turn blue with May Grunwald staining, orange reddish with PAS and black with Sudan III and osmic acid. The sea-blue histiocytes are preferably located at the bone marrow, liver and spleen and less frequently in lymph nodes, lungs and some other organs. The prognosis is variable: fatal in the central nervous system location, relatively mild in cases of spleen and bone marrow location. The possibility of complicating hepatic cirrhosis and/or pulmonary fibrosis is always present. Seven cases are described in this paper, 4 of them family related. Acute myelomonocytic leukemia in one case and histioimmunoblastic lymphoma in another were complications not yet reported in the literature.