Pulmonary microthromboembolism is one of the serious complications found in patients with thalassemia. The pathogenesis is undetermined. The thrombotic risk in 44 patients (26 males, 18 females) with beta-thalassemia/hemoglobin E disease and without clinical symptoms of thrombosis were studied. The age ranged from 3-19 yr (X +/- SD = 10 +/- 4). Neither of them had chronic hepatitis B infection. They were divided into three groups according to clinical manifestations as follows: (1) Mild form (n = 12). They did not require blood transfusion. The mean +/- SD of hematocrit was 23.3 per cent +/- 2.3; (2) Severe form (n = 19). They required frequent blood transfusion. The mean +/- SD of hematocrit was 17.7 per cent +/- 1.5; (3) Severe form with splenectomy (n = 13). They seldom required blood transfusion. The mean +/- SD of hematocrit was 21.8 per cent +/- 3.5. Most of the patients had delayed growth. They had high serum ferritin reflecting iron overload status which was prominent in the severe groups (group 2 & 3). The prothrombin time and serum albumin were slightly decreased, and the serum alanine transaminase were slightly increased; all of which reflected mild alteration of liver function. The plasma AT III, PC and PS antigen in the three groups were similar. The mean +/- SD of AT III antigen was 106.7 per cent +/- 22.2 which is normal. The mean +/- SD of PC antigen was 44.2 per cent +/- 14.2 and PS antigen level was 77.2 per cent +/- 17.8.(ABSTRACT TRUNCATED AT 250 WORDS)