The role of hereditary antithrombotic protein defects in juvenile deep vein thrombosis (DVT) was evaluated. Fifty six young patients (age <45 yr) with doppler-proven DVT were investigated for the presence of resistance to activated protein C (APC-R), lupus anticoagulant (LA), anticardiolipin antibodies and deficiencies of protein C, protein S, ATIII activities. Fifty nine normal healthy individuals served as controls. APC-R was observed to be the commonest defect underlying the Indian DVT as seen in 39.2% of patients followed by elevated ACA (5.3%), PAI (2.8%), presence of LA (2.8%) and reduced ATIII levels (2.8%). None of the subjects had protein C or S deficiency. APC-R was associated with ATIII deficiency in one case, and elevated ACA in two cases. In two subjects, APC-R was associated with elevated PAI levels. Patients with more than one prothrombotic factor had a higher prevalence of pulmonary thromboembolism, suggesting that the thrombogenic potential of APC-R is enhanced by the presence of coexisting hereditary or acquired prothrombotic defect.