10 asymptomatic young male patients with moderate splenomegaly detected at a routine examination are presented. The history and clinical examination failed to reveal the aetiology of the splenomegaly. Further investigations, including screening for blood dyscrasias, clotting abnormalities and reticuloendothelial abnormalities were likewise unrevealing. Liver biopsies, rectal biopsies for bilharzia and bone marrow aspirates for Gaucher's Disease were found to be normal. Serology for malaria and Ebstein Barr Virus infection was also negative. Positive immunofluorescent tests for IgG antibodies specific for cytomegalovirus were found in 5 patients. We consider that these patients have splenomegaly which is not of a specific nature, but may be associated with a severe antigeneic response to the previous cytomegalovirus infection. In view of the otherwise negative findings these patients should be considered to have ‘True Idiopathic Splenomegaly’, a term which would indicate the benign nature of the splenic enlargement. This diagnosis should be considered in the differential diagnosis of asymptomatic patients who have splenomegaly of undetermined ori