This specimen is from a 3 yr. old boy who presented with abdominal mass. The right kidney is completely replaced by thin-walled cysts filled with serous fluid. The cysts were lined by flattened, cuboidal, and hobnail cells. Small foci of blastema and abortive tubules were seen in the septa. Case courtesy of: Dr. Lamiaa Rouas and Dr. Nadia Cherradi, Dept. of Pathology, Mohamed V University School of Medicine, Rabat, Morocco.