Seven cases of ‘thorotrast kidney’ after retrograde pyelogram with reflux are dealt with. In five instances cancer of the kidney developed, four of them obvious renal pelvic carcinoma between 14 and 41 years after the thorotrast pyelogram. In some cases the contrast deposits were misinterpreted and in one case the follow-up was also inadequate. There is a high risk of malignancy in connection with ‘thorotrast kidneys’. These cases should be followed up with intravenous pyelogram and exfoliative cytology from renal pelvis. Provided these tests are negative, nephro-ureterectomy should be put off.