Despite numerous well-described causes of stroke in infancy and childhood, a significant proportion remains unexplained. Venous thromboembolism is a common complication in adult patients undergoing surgery, and after severe trauma, but not in otherwise healthy children less than 10 years old. However, it may also occur spontaneously without recognizable cause. It has been known for a long time that some patients are particularly prone to venous thrombosis and in recent years great efforts have been made to identify the risk factors. The attention of haematologists has been focused on the possibility that certain abnormalities of coagulation may be associated with a tendency to thrombosis, but only in a few instances a clear causal relationship has been established. One such example is a deficiency of antithrombine III, but such a deficiency has hitherto been recognized as a cause for thrombosis in children only in very particular circumstances. We present two young children with stroke of which one was purely ischemic and the other ischemic with secondary hemorrhage. Both our patients showed an AT III deficiency. Patient one also had a cyanotic congenital heart disease with right to left shunting which made cerebral embolism originating from a thrombus in the iliac vein possible to occur. We consider her hematocrit values too low to be a predisposing factor for this thrombosis. AT III deficiency may be caused by several different mechanisms. Either it exists as a congenital (hereditary) or as an acquired disorder. In patient two the family history was positive for hereditary AT III deficiency.(ABSTRACT TRUNCATED AT 250 WORDS)