Resistance to activated protein C (APC-R) is the leading cause of deep vein thrombosis (DVT). Deficiencies of protein C (PC) or its cofactor protein S (PS) are less frequent. Resistance usually results from nucleotide substitution (1691 GA) in the factor V gene (Leiden mutation). We searched for APC-R and the Leiden mutation (FVL) and measured the activities of PC, PS, antithrombin III (AT III) and Fb levels in patients with DVT. The results were analyzed against a history of thrombosis (idiopathic, risk factor related). We enrolled 29 patients aged 50 years or younger, with first symptoms of thrombosis detected before the age of 40 years. The control group consisted of 25 healthy volunteers of similar age. APC-R was established using Accelerimat (bioMerieux) assays. APC-R was diagnosed when the normalized sensitivity coefficient "r" was < 0.9. FVL was detected with the SSP-PCR (sequence specific primers-polymerase chain reaction) method. Abnormal APC-R "r" values were found in seven DVT patients (24%). Heterozygous (G/A genotype) form of the Leiden mutation was confirmed in six of them (all had a history of recurrent thrombosis). FVL carriers demonstrated lower PC levels in comparison with controls and DVT without FVL. Eight patients (27%) had PS activities below the cut-off point (60%). The deficiency in six of them was not associated with other abnormalities. Patients with recurrent thrombosis had markedly higher concentrations of Fb (usually without FVL) and reduced AT III activities. The study has shown that the APC-R test is useful for FVL screening. The Leiden mutation, elevated levels of Fb and reduced activities of PC are the main factors predisposing to DVT and its recurrence. Reduced activity of PS is usually an isolated abnormality tending to predispose to a single DVT episode.