431 patients with thrombosis of different venous system were evaluated for underlying acquired and inherited prothrombotic states. Associated acquired risk factors were observed to be present in 28.7% patients and possible inherited in 32.3%, in the rest, no cause could be identified. Major acquired risk factors included coexistence of liver disease (12.2%), oral contraceptives (4.1%), puerperium (2.5%), malignancy (2.3%) and lupus anticoagulant (2%). Low levels of protein C were detected in 21.1% and of which 11.3% were attributed to acquired factors. Protein S deficiency was found in 19.0% and of these 10.4% cases were associated with acquired risk factors. Antithrombin III (AT III) deficiency was detected in 6.4% of patients, of which 4.8% were secondary to acquired factors. In the rest, deficiency of protein C, protein S and AT III were attributed to inherited factors as no associated acquired risk factor was present. Activated protein C resistance (APC-R) was present in 12.5% cases.