The unusual case of a boy with a stroke occurring at three years of age, transient reduction in protein C activity and high concentrations of antiphospholipid antibodies (APA) is described. APA or Lupus Anticoagulant (LA) were found in 7 of 11 relatives studied out of three different generations. In addition, antigenic Factor (F) XII deficiencies or borderline values were found in the propositus and 2 relatives. Evidence for F. XII inhibitors was found in the propositus, one of his brothers and both of his parents. Whether F. XII inhibitors in patients with APA and/or LA are pathophysiologically relevant in vivo or if they are only an in vitro phenomenon remains to be elucidated. It is reasonable to believe that the main laboratory pathology (APA and/or LA activity) in antiphospholipid syndrome is related to the clinical picture of a hypercoagulable state. There is evidence from the literature that deficiency or inhibition of F. XII might contribute to a prothrombotic state through impairment of the fibrinolytic system. There is also evidence that APA are able to reduce protein C activation. From a clinical point of view, it seems that hypercoagulability in our patient was controlled by low-dose aspirin therapy (75 mg/d). In conclusion, this case seems to support the idea of a genetic predisposition for the development of APA and/or LA. The related disturbances of the coagulatory, anticoagulatory and fibrinolytic systems might contribute in different ways to the prothrombotic state seen in patients with "antiphospholipid syndrome", eventually resulting in possible venous thrombosis or arterial thrombosis with corresponding ischaemic lesions.(ABSTRACT TRUNCATED AT 250 WORDS)