In spite of improvements in chemical structure, contrast media assisted X-ray examination is still the third leading cause of hospital-acquired acute renal failure. An increase >50% or >88 micro mol/L in S-creatinine is a clinically important acute renal failure. The peak in S-creatinine occurs within 2-5 days after exposure. The frequency of oliguria, transient or permanent haemodialysis is unknown. The cause is a hypoxic tubular injury due to vasoconstriction with release of free oxygen radicals. Major risk factors are prior renal insufficiency and diabetes mellitus. Minor risk factors are congestive heart disease, dehydration, hypotension, hypoxia, amount of contrast, ionic and high osmolar contrast, repeated examinations at short intervals, abdominal examination, and perhaps age, smoking, hypercholesterolaemia, and use of Non-Steroidal Anti inflammatory Drug. Prevention seems possible by omission or reduction of contrast, ameliorating predisposing factors, saline hydration 24h before and after exposure, and 600 mg acetylcysteine orally twice daily 24h before and after exposure. A three-day treatment with 20mg nitrendipine daily, starting 1 day before examination may also be preventive. The present research is unfortunately characterised by small numbers, lack of clinical important renal failure, and lack of long term results. The latter may be important after new data indicate that radiation may trigger a chronic oxidative process through a similar pathway.