The edict to contain costs and meet goals imposed by DRG remuneration policies mandates the work-up of asymptomatic renal mass lesions on an outpatient basis. This proved feasible in 98 per cent of patients. The vast majority of such mass lesions (82 to 90 per cent) is diagnosed with acceptable confidence by computed tomography and sonography alone. For a shrinking group of such patients, yet still 16 to 18 per cent, guided percutaneous aspiration biopsy is necessary to affirm the diagnosis. However, this technique has been refined during recent years to incorporate the use of thin needle equipment and can now be performed on an outpatient basis without significant risk of morbidity. For diagnosing hyperdense inflammatory and infected renal cysts, guided percutaneous aspiration is recommended as the most effective method. This procedure should take precedence over surgical exploration because it can diagnose and provide pertinent bacteriologic information that may determine the course of therapy. In many instances inflammatory cysts or even silent renal abscesses are diagnosed by a percutaneous aspiration technique that is then expanded to serve therapeutic purposes such as percutaneous drainage. Even these procedures can be performed safely on an outpatient basis provided the patient is followed closely. Because complications of percutaneous aspiration procedures are extremely rare, the procedure can be used safely on an outpatient basis. The impact of magnetic resonance imaging on the diagnosis of asymptomatic space-occupying lesions of the kidney is as yet not fully determined; however, this method appears promising for diagnosing some of the refractory lesions such as hemorrhagic cysts, aneurysms, or arteriovenous malformations.