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The development of a multifactorial coagulopathy after massive transfusion is a well-recognized clinical problem that is almost always accompanied by hypothermia. The purpose of this study was to investigate the isolated effect of alterations of temperature on the integrity of the coagulation cascade. Prothrombin times and partial thromboplastin times were each performed 15 times on samples of pooled normal plasma at the temperatures of 37°C, 34°C, 31°C, and 28°C, as well as 39°C and 41°C.Mean prothrombin time results increased from 11.8 ±PT 0.3 (SD) secs at 37°C to 12.9 ±PT 0.5, 14.2 ±PT 0.5, and 16.6 ±PT 0.2 secs at 34°C, 31°C, and 28°C, respectively (p < .001 for each). Partial thromboplastin time determinations increased from 36.0 ±PT 0.7 (SD) secs at 37°C to 39.4 ±PT 1.0, 46.1 ±PT 1.1, and 57.2 ±PT 0.6 secs at 34°C, 31°C, and 28°C, respectively (p < .001 for each). Both prothrombin time and partial thromboplastin time determinations were only minimally shortened at hyperthermic temperatures.The series of enzymatic reactions of the coagulation cascade are strongly inhibited by hypothermia, as demonstrated by the dramatic prolongation of prothrombin time and partial thromboplastin time tests at hypothermic deviations from normal temperature in a situation where factor levels were all known to be normal. Clinicians who deal with critically ill massively transfused hypothermic patients all recognize the inevitable appearance of a coagulopathy that has a multifactorial origin. Unless specifically considered, the contribution of hypothermia to the hemorrhagic diathesis may be overlooked since coagulation testing is performed at 37°C, rather than at the patient's actual in vivo temperature.