Sea-blue histiocytosis, also known as inherited lipemic splenomegaly, is an extremely rare condition characterized by elevated triglyceride levels (hypertriglyceridemia) and an enlarged spleen (splenomegaly). The disorder is so named because certain white blood cells, known as histiocytes, appear bright blue when stained and viewed under the microscope. Additional signs and symptoms may include a low platelet count (thrombocytopenia), liver function abnormalities, and heart disease. It is one of a group of related fat (lipid) disorders caused by certain changes in the APOE gene. The genetic change associated with this condition is inherited in an autosomal dominant manner though other factors, such as a patient's gender, the patient's lipid levels, and the genetic makeup of the other APOE gene may play a role in how the condition is expressed. There are currently no formal treatment guidelines. Management may involve the coordinated care of several different specialists including cardiologists, gastroenterologists, and hematologists. Patients with splenomegaly should be careful to avoid contact sports. Removal of the spleen (splenectomy) has been reported to make the condition worse.

Sea-blue histiocytes can also be a secondary finding associated with a wide range of disorders, including myelodysplastic syndromes, lymphomas, chronic myelogenous leukemia, idiopathic thrombocytopenic purpura, Niemann-Pick disease, and Norum disease. In these cases, treatment depends on the underlying disorder.