This study reports on the frozen stability of all commonly measured coagulation proteins in normal citrated plasma: activated partial thromboplastin time, prothrombin time (%), thrombin time and fibrinogen (Clauss); clotting assays for factors II, V, VII, VIII, IX, X, XI and XII; functional assays for protein C (clotting), protein S (clotting), antithrombin (chromogenic) and plasminogen (chromogenic); and immunological assays for von Willebrand factor and D-dimer. All these factors listed are stable for up to 3 months if frozen at −24°C or lower. At −74°C, all these factors (allowing for 10% variation) were stable for at least 18 months, most were stable for 24 months. The number of proteins showing > 5% variation over baseline after 6 months storage indicates that some decay does occur even at −74°C. There was no clear advantage in snap freezing at −74°C and then storing at −24°C over both freezing and storing at −24°C; therefore, the freezing process itself is not responsible for the loss of stability. The best stability, especially at −24°C, was obtained when small samples (1 ml) were stored in screw-cap tubes with a minimum dead space. The decrease in stability of the coagulation proteins directly correlates with the effect of temperature and time.