A proper laboratory diagnosis of inherited thrombophilia due to defects of the coagulation system may be obtained only by a stringent definition of the diagnostic reference ranges taking into account the influence exerted by major physiological variables. To this purpose, we analyzed the data for Protein C, Antithrombin III, Heparin-Cofactor II and plasminogen coming from the first 4,000 subjects enrolled in the VITA Project. This is the first study that allows the establishment of reference ranges in a non-selected, active population using stringent standardization of clinical and laboratory measurements and multivariable regression techniques for data analysis. Serum triglycerides and total cholesterol, together with plasma fibrinogen were found to influence the functional plasma level of the four considered proteins. Menopause increased AT-III concentration, while pill use increased Heparin-Cofactor II and plasminogen. PT and PTT ratio, gender, age, smoking, body-mass index, HDL cholesterol and blood group had minor effects. The effect of these variables should be taken into account for both clinical and epidemiologic purposes, using appropriate reference ranges or covariance analysis for adjustment.