Summary.Renal cysts in the cortex of the monkey kidney were observed by scanning and transmission electron microscopy, in an attempt to determine the three-dimensional structures of the epithelial cells in these cysts.The cysts were composed of a thin wall which limited a large spherical space containing a jelly-like substance and some wandering cells. Rudimentary tufts of glomerular capillaries and/or a mound-like swelling were present on the inner surface of the cyst wall, and the inner surface of the cysts was lined with an epithelium with pedicels. The epithelial cells were podocytes, whose surface structure was similar to that on the glomerular capillary in the normal renal corpuscle.The terminal foot processes were characteristically interdigitated not only on the glomerular capillaries, but also on the mound-like swelling and concave surface of the parietal portion of the cyst.A discrete, smooth-surfaced area was found on the side opposite to the glomerulus or swelling.This area was covered by squamous epithelial cells.At the peripheral zone of the smooth-surfaced area, elongated epithelial cells were demonstrated. Our findings indicate that the podocyte layer of the cyst corresponds to the internal leaflet of Bowman's capsule, and the squamous cell layer to the external leaflet. The moundlike swelling covered by podocytes is considered to represent a stage of possible evagination of the tufts of glomerular capillaries.There was no evidence of an opening of the urinary tubule in the cyst wall.