In 47 patients the posterior radionuclide views of the spleen in supine and prone positions were evaluated for difference in splenic contour and calculated splenic weight as the patient's position was changed from supine to prone. A significant change in splenic contour was found in almost 50% of the cases. Values for the mass of the spleen varied by a mean of 36% with a range of plus 195 to minus 79%. The differences are attributed primarily to changes in the position of the spleen and liver within the abdomen.