In two children, a 9 year-old boy and a 10 1/2 year-old girl, who presented with polycythaemia as the only symptom, the expected renal tumour was only found after exclusion of all other causes of polycythaemia. The delay in diagnosis was caused by technically inadequate intravenous urograms, which were erroneously passed as normal. In one child low kv X-ray exposition of the kidneys led to the diagnosis of a renal tumour. In the other child high-dose urography and tomography gave the indication for selective angiography. Normalization of the red blood count postoperatively verifies the connection between preoperative erythrocytosis and the renal tumour. Histologically both cases proved to be renal adenomas, which are extremely rare in childhood.