In this review an interim analysis was made of all clinical studies performed with low-molecular-weight heparin(oid)s (LMWH) up to January 1987. Thus far, many experimental studies on LMWH show these substances to have an increased benefit/risk ratio concerning efficacy and bleeding as compared to standard heparin. In man, this increased ratio was verified in some small open studies. However, until the present day, controversy still exists. Some of the difficulties in assessing the results of different investigations include the existence of various kinds of LMWH and the lack of a consensus as to the unitage in which they are expressed. Furthermore, no international agreement exists as to the test measuring dosage/effect and its standard. In the following review all published clinical studies were analyzed for patient groups. Important issues concerning controversy with regard to efficacy, bleeding risk, diagnosis, and testing are discussed. In general, it may be concluded that LMWH have an antithrombotic potential comparable to that of heparin, and that the benefit/risk ratio i.e., decreased bleeding potential when compared to standard heparin, as found in animal experiments, has not been clearly demonstrated in man.