Clinically significant splenomegaly is frequently not apparent upon routine physical and roentographic examinations. In addition, it may be difficult to. differentiate an enlarged spleen from other masses that may appear in the left-upper quadrant. Splenic-imaging techniques have proven to be valuable in the recognition of splenomegaly, in the differential diagnosis of left-upper quadrant masses, and for detecting space-occupying lesions within the spleen. Two functions of the spleen provide the basis for the currently available spleen-scanning techniques: (1) Appropriately damaged erythrocytes are preferentially sequestered in the Splenic sinusoids; a number of radiopharmaceuticals are available that can be used to label the erythrocytes to permit localization of this organ by external scanning techniques. (2) The reticuloendothelial cells of the spleen, as well as those of other organs, accumulate intravenously injected colloidal particles by phagocytosis; colloids prepared from the short-lived generator-produced gamma-emitting nuclides, such as 99mTc and 133mIn, provide high photon images with a smaller radiation dose to the patient than that associated with the use of most erythrocyte methods for spleen scanning. The radioactive colloids make it possible to do simultaneous scanning of the liver and the spleen. Although many methods are adequate for a majority of clinical problems, colloidal preparations labeled with these short-lived nuclides are preferred for routine use. The use of the gamma camera permits obtaining multiple views of the spleen that often provide more structural detail than the conventional posterior and left lateral views alone.