An abnormality in platelet aggregability or fibrinolysis, namely elevated activity of plasminogen activator inhibitor-1 (PAI-1), has been recently documented in patients suffering from Klinefelter's syndrome associated with leg ulceration without underlying venous insufficiency. To determine whether increased PAI-1 activity is a general feature of Klinefelter's syndrome, or more specifically associated with leg ulceration, we investigated PAI-1 influencing parameters and PAI-1 activity in two groups of patients: (i) Klinefelter patients suffering from leg ulceration (n = 7); and (ii) Klinefelter patients without leg ulceration (n = 6). On analysing PAI-1 influencing parameters such as age, body mass index, triglycerides, C-reactive protein, testosterone, smoking behaviour, the presence of diabetes mellitus, and arterial hypertension, respectively, we found no statistically significant differences between the two groups. However, PAI-1 activity in group 1 was highly significantly elevated compared with that in group two patients (P < 0.005). We conclude that (i) PAI-1 activity is not elevated in Klinefelter's syndrome in general; (ii) elevation of PAI-1 activity in patients suffering from Klinefelter's syndrome does not appear to be secondary to PAI-1 influencing parameters; and (iii) elevation of PAI-1 activity may play a crucial role in the pathogenesis of leg ulceration in Klinefelter's syndrome. Therefore, a therapy for leg ulceration in Klinefelter's syndrome that aims to return the elevated PAI-1 activity to normal should be explored.