Protein C (PC) is the main anticoagulant protein of the hemostasis system. It can inhibit the blood clotting cascade before the formation of a thrombus, while its concentration can decrease significantly during strong activation of blood clotting. The PC concentration was found to decrease during systemic lupus erythematosus (SLE) (with a median of 75%) and depended heavily on the inflammation index. It was also associated with the accumulation of soluble fibrin monomeric (SFMCs) (with a median of 7 µg/mL). A low PC level was detected during severe ischemic heart disease (IHD) (with medians of 60 and 63%, respectively). These pathologies also were associated with clotting activation. During abdominal aortic aneurysm (AAA), the PC level in blood plasma before surgery was found to range from 40% to 119%. A decrease in the PC level in the blood plasma of patients with AAA before surgery, lower than 78%, was associated with high blood loss (more than 1.5 L). A decrease in the PC level can lead to an imbalance between coagulation and anticoagulation. Thus, during the treatment of complex pathologies associated with the activation of coagulation, specific attention should be paid not only to classic markers of thrombus formation but also to the state of the anticoagulant link.