This review studied 9 cases of young or middle-aged patients with protein C or protein S deficiency presenting with thrombotic events in the venous or arterial system. The clinical features were consistent with those described in the literature. Affected family members with similarly low levels were also detected in family studies, some of whom did not have any thrombotic history. Serial measurement of protein C, factor II and factor X at various International Normalization Ratio (INR) levels showed a correlation between suppression of these vitamin K dependent factors and degree of anticoagulation. Furthermore, there was a distinguishable difference in these indices (protein C level, protein C: factor II, protein C: factor X ratio) in warfarinized protein C deficient patients when compared with warfarinized patients who were not protein C deficient. This provides a means in diagnosing protein C deficiency even in warfarinized patients. Such a pattern was not found for protein S deficient patients, which may be due to the assay method used.