At the instance of 4 patients the differential-diagnostic difficulties between advanced malignant renal tumours and the so-called septic kidney are demonstrated. In fever conditions in connection with unclear urographic findings, in particular with the so-called dumb kidney, it is right primarily to search for a septic renal disease. In spite of urgency of diagnosis and above all of the therapeutic consequences resulting from this the presence of an advanced malignant renal tumour must be included in the differential diagnosis also when there are no leading tumour symptoms. This demands the rapid use of a highly specialised radiologico-urologic diagnostics. Many clinical and paraclinical findings are differential-diagnostically without importance, since they may be typical for the two pictures of the disease. A certain preoperative diagnosis should be achieved above all on account of the differentiated operative approach and the operative way of access.
