Hereditary Antithrombin III (AT III) deficiency was found in a large Hindustani family, living partly in the Netherlands, partly in Suriname. Of 201 members investigated 35 were found to affected: AT III activity (chromogenic substrate) and AT III antigen (immuno-electrophoresis according to Laurell) were about 45 %. Analysis of this fanily clearly demonstrated the autosomal dominant inheritance of the condition. Six non-investigated members (1 living, 5 non-living) were diagnosed as being affected on the basis of affected offspring.Seventeen affected members had no signs of thrombo-embolic(TE) processes (age group 0-10 years old, n=2; 11-20, n=5; 21-30, n=4; 31-40, n=4; 41-50, n=2). Thirteen showed clinical or proven signs of TE processes (first time in age group 0-10 years old, n=0; 11-20, n=l; 21-30, n=G; 31-40, n=4; 41-50,n=l; 51-60, n=0; 61-70, n=l). No clinical information is yet available on the remaining affected members. Deep venous thrombosis (DVT) occurred in 9 patients (age group 21-30, n=5; 31-40, n=3; 61-70, n=l). Triggering factors were none 4, surgery 1, oral contraceptives and preg- nancy4. Pulmonary embolism occurred in 6 patients (2 clinical, 4 proven) and was fatal in 4; ages were 19, 21, 26, 37, 48 and 68 years old. Pregnancy was uncomplicated in 3 women (total of 4 pregnancies), one of these women was treated prophylactically with anticoagulants during pregnancy (1 pregnancy). Two women (9 pregnancies) had a thrombotic episode (1st and 3rd pregnancy respectively) and 1 woman died suddenly 7 days after her 7th childbirth. DVT occurred in 2 of 4 women who used oral contraceptive pills.In some symptomless patients (age 22, 26, 32, 33, 40 years old) impedance plethysmography (n=5), 125I-fibrinogen leg scanning (n=3), 125I-fibrinogen T½(n=3) and 51C-platelet survival (n=l) were normal