Marble spleen disease (MSD) is a contagious disease of captive-reared ring-necked pheasants (Phasianus colchicus) characterized by variable mortality and high morbidity. The etiologic agent is a type II avian adenovirus closely related to hemorrhagic enteritis virus of turkeys and splenomegaly virus of chickens. Gross lesions of MSD consist of enlargement and mottling of the spleen and pulmonary edema. Microscopic alterations are most pronounced in the spleen and are characterized by reticuloendothelial (RE) cell hyperplasia and lymphoid necrosis. Intranuclear viral inclusions occur within hyperplastic RE cells. Serologic detection of MSD is routinely accomplished using an agar gel precipitin test. Oral vaccination with hemorrhagic enteritis virus or avirulent strains of MSD virus is effective in preventing MSD. There is no specific treatment available for MSD but supportive care, strict sanitation and good management practices are helpful in reducing mortalities and limiting the spread of an epizootic.