The two components of biological variability are interindividual variability, which is the variability due to the heterogeneity of physiologic influences among subjects, and intraindividual variability, which is due to the variability in the same individual over time. Analysis of biological variation is crucial for estimating the critical difference, which corresponds with a threshold suggestive of a statistically significant difference between two consecutive results of a laboratory parameter in the same subject and is therefore unlikely attributable to casual (random) oscillation of values. Studies on biological variation of tests of hemostasis are outdated, and the published results should be confirmed by using modern, fully automated methods. Biological variation for coagulation screening tests (prothrombin time and activated partial thromboplastin time) is low and comparable with the values registered for hematologic parameters. However, the index of individuality (ratio between intraindividual and interindividual variability) suggests that the usual preoperative screening for coagulation disorders is influenced by the between-subjects variability. Thrombin time is very constant within and between subjects. Proteins such as fibrinogen, clotting factors, and antithrombin show a low biological variability. In contrast, fibrinolytic parameters, such as plasminogen activator inhibitor 1 and fibrinopeptide A, show very high variability, and their interpretation in the clinical setting must take this into consideration.