AT III activity and concentration were measured in 36 patients (mean age 65.5 yrs, range 43-77 yrs) with ischaemic stroke within maximally 48 h of the acute event. In 12 patients (= 33%) AT III activity was reduced below 18.4 IU/ml: 50% of these patients showed normal and 50% reduced AT III concentration of less than 22 mg/dl. In 15 patients AT III activity and concentration were measured in the acute phase on admission to hospital and 12 months later. In the acute phase, AT III activity was reduced when compared with AT III concentration (y = 0.19 chi + 15.5) and did not correlate with the latter. 12 months later, however, AT III activity and concentration correlated significantly (r = 0.92; p less than 0.001) and the regression line was steeper (y = 0.8 chi). During the acute phase of ischaemic stroke, intravascular coagulation is evidently increased and inactive AT III-thrombin complexes are formed, whereby the concentration of active AT III decreases. A patient with progressive stroke and reduced AT III activity of 14.2 IU/ml was therefore substituted with AT III concentrate. The further neurological course was favourable.