Many unrelated clinical conditions that are associated with significant decreases in antithrombin III have been summarized. In addition, prospects for future therapy with antithrombin III concentrates have been discussed. The clinical indications for antithrombin III determinations, including the reasons for performing the assay and those specific patient populations subjected to antithrombin III assays in this author's clinical practice, have been outlined. It is to be anticipated that with the new general availability of simple reliable antithrombin III assay systems using synthetic substrates that more and more populations will be adequately studied and clinical indications for the use of antithrombin III determinations and concentrates may become more firm or may change.