A large nonparasitic, non-neoplastic, benign cyst of the spleen in a 24-year-old woman was diagnosed by conventional radiography, radionuclide scintigraphy, and computed tomography. Round contour, sharp margins, and homogenous content of low attenuation value are characteristic computed tomographic features. The histologic study of the cyst and information obtained from the literature provide evidence to support the hypothesis that the cyst develops from an embryonal rest of the primitive coelomic mesothelial cells in the spleen. Mesodermal cyst is therefore a more appropriate designation for such a cyst. © 1983.