One hundred and twenty-six patients with renal cell carcinoma were treated by nephrectomy between 1985 and 1988. They were classified into three groups: group A: 47 patients in whom ultrasonography revealed the renal cancer in the absence of any suggestive clinical signs of tumour; group B: 63 patients who presented with clinical urological signs suggestive of the tumour; group C: 16 patients who presented with general signs leading to the diagnosis. In group A, 50% of the tumours measured between 5 and 10 cm, 51% were located at the lower pole of the kidney and 83% were stage T2. Sixty-six percent of cancers in this group were situated on the right side, indicating that left renal cancers are missed in 16% of cases. In group B, 60% of the tumours measured between 5 and 10 cm, 44% were located at the lower pole and 56% were stage T2. In group C, 60% of the tumours measured between 5 and 10 cm, 50% were located at the upper pole of the kidney and only 38% of the tumours were still stage T2. We can conclude that incidental detection reveals renal tumours at a relatively limited stage (83% of T2), with dimensions smaller than those of the other groups. It is therefore essential for radiologists, ultrasonographists and urologists to investigate the left lumbar fossa and the upper pole of both kidneys very carefully during abdominal or vesico-prostatic ultrasound examinations.