The influence of 51 preoperative, peroperative and postoperative variables on the development of serious acute renal failure (ARF) following open heart surgery was studied. Although a large number of significant variables was found, a logit-model with only 2 explanatory variables showed an almost perfect fit. With this model the chances of serious ARF up to 90% were estimated. The results suggest that a critical circulation is the main cause of serious ARF. Further-more, a reduced ability to cope with a critical circulation without renal failure plays an important role in the pathogenesis. There is a higher risk of serious ARF for patients older than 70, especially when circulatory Support with dopamine is needed.