WE DESCRIBE FOUR PATIENTS WITH NIEMANN Pick disease type C (NPC), in whom the presentation was isolated splenic enlargement; this remained the only abnormality for a number of years. Diagnosis can be suggested by either finding abnormal storage material in a tissue biopsy specimen or by showing a modest elevation in plasma chitotriosidase activity. In patients with suggestive abnormalities, filipin staining of a skin fibroblast sample should confirm the abnormality in cholesterol trafficking. Formal esterification studies and mutation analysis should also be performed, especially if prenatal testing is to be performed in subsequent pregnancies. If the diagnosis is not considered and established, the family are at risk of having further affected children. Investigation of patients with isolated splenomegaly is not complete until NPC has been excluded.