Four cases of cystic renal adenocarcinoma in adults free of renal dysfunction are reported. The authors propose a diagnostic approach for atypical cysts that takes the new imaging techniques into account (ultrasonography and CT scan initially, MRI subsequently). A practical therapeutic strategy is also described: because intraoperative histologic diagnosis is difficult partial nephrectomy to allow an intraoperative study of the entire cyst is advocated; extensive nephrectomy, which is the only procedure that ensures total exeresis of the carcinoma, is performed either immediately if the intraoperative study of the cyst or of its aseptic necrotic and hemorrhagic content is positive, or later if subsequent histologic studies reveal a malignancy.