The antithrombin III (AT III) activity and the AT III concentration were investigated in 62 consecutive patients with acute myocardial infarction (AMI). To identify the reactant pattern of AT III in postaggressive situations, we also determined labile and acute phase proteins. Firstly, 29 patients were nourished orally and then 33 patients were fed by i.v. hyperalimentation (additional caloric intake of approximately 1,000 cal). AT III activities and concentrations as well as prealbumin and retinol-binding protein decreased concomittantly and significantly whereas haptoglobin, C-reactive protein and fibrinogen increased significantly after AMI. The changes cannot be interpreted as being alterations of the haematocrit. The alterations of AT III correlated significantly with the changes of labile proteins but not with the acute phase reactant proteins. The AT III decrease in the postinfarctional phase may promote a prethrombotic state. In addition it can be concluded from our results that AT III reacts as (nutritive-dependent) labile protein, which is lowered in postaggressive situation and does not increase as an acute phase reactant. This is in accordance with results from recent animal experiments.