Mild hyperhomocysteinaemia is an established risk factor for deep vein thrombosis (DVT); few data concerning its potential interaction with thrombophilic genotypes are available at the present time. We investigated 121 thrombosis-free individuals and 111 patients with at least one objectively confirmed episode of DVT. A thrombophilic condition (deficiency in antithrombin, protein C and S, factor V Leiden, prothrombin G20210A) was detected in 25.2% of the patients; mutant factor V or prothrombin genotypes were present in 6.6% of the controls. Hyperhomocysteinaemia was found in 14.4% of patients and 3. 3% of the controls, with a 3.7-fold increase in risk for DVT (95% CI 1.1-12.3). Adoption of different cut-off levels for definition of hyperhomocysteinaemia did not substantially change the magnitude of the risk. Carriership of both hyperhomocysteinaemia and factor V Leiden or prothrombin G20210A was detected in 2.7% of patients for each combination and in none of the controls. An approximate estimate of 30-fold increased risk in carriers of both hyperhomocysteinaemia and factor V Leiden and 50-fold increased risk in carriers of both hyperhomocysteinaemia and prothrombin G20210A was calculated, suggesting a synergistic interaction between hyperhomocysteinaemia and such thrombophilic genotypes. Yet statistical analysis is highly unstable due to the small number of individuals with combined defects. Further investigations on large series of patients are needed.