Sea-blue or ceroid histiocytosis is a storage phenomena associated with a variety of conditions especially abnormal lipid metabolism and particularly hyperlipoproteinemia. It is characterized by histiocytic proliferation in the bone marrow and spleen, contain-ing sea-blue inclusions by Romanovsky stain. The present case is a 1 ½ year-old Korean boy who had marked enlargement of the spleen which was eventually removed. Aspirates of the bone marrow and histology of the spleen disclosed an enormous proliferation of histiocytes containing numerous cytoplasmic inclusions which stained sea-blue with Wright stain, was strongly positive to PAS and weakly positive to oil red-O and Sudan black B in frozen and in paraffin embedded section. Ultrastructually histiocytes were marked1y hypertrophic and contained numerous cytoplasmic inclusions which showed three distinct types and conglomeration of all three types, presumably representing age or maturation steps of the inclusions. The ear1y type consisted of a high electron dense core or deposits within a low electron dense matrix, evolving into homogeneous moderately electron dense inclusion and finally a well developed finger print-like internal structure. Analysis of the plasma lipid disclosed type IIb hyperlipoproteinemia. Types of hyperlipoproteinemia previously reported in association with sea-blue histiocytosis were type-I, III, IV and V, and this is the first case of type IIb hyperlipoproteinemia.