It is the purpose of this paper to review our present knowledge about uremic toxicity, with a special emphasis on the methods that have been used to try to resolve this problem. More and more, sophisticated methods become available for the study of uremic toxicity. Many of these techniques are complicated and expensive, and have only a limited distinctive capacity. The HPLC method, however, is refined and allows a clear distinction between an impressive number of components. Moreover, there is a remarkable difference between the HPLC method appears to us as a valuable technique for the further study of uremic toxicity. At the moment, it is not possible to define each of the recognized products as uremic toxins. The aim should be, however, to eliminate retention products by dialysis in a way that the post-dialysis HPLC pattern resembles normal serum. Together with this strategy, the search for reliable markers of adequate dialysis should be continued.