The plasma metalloprotease ADAMTS13 (A Disintegrin And Metalloprotease with ThromboSpondin type 1 motif 13) cleaves prothrombotic ultralarge multimers of the platelet-adhesive protein von Willebrand factor (ULVWF) into less active multimers that promote haemostasis in injured blood vessels. When the enzyme is dysfunctional or undetectable, circulating ULVWF may cause massive intravascular aggregation of platelets and thrombotic thrombocytopenic purpura. This study compared ADAMTS13 antigen and activity in a large set of plasmas collected from subjects with various conditions of health and disease, most of which were associated with an increased thrombotic tendency. Pathological conditions were liver cirrhosis (n = 90), inflammatory bowel disease (n = 44) and cardiac surgery (n = 30). Healthy conditions were pregnancy (n = 42), oral contraceptive intake (n = 33) and the neonatal state (n = 41). Normal individuals of different ages were taken as controls (n = 132). The antigen assay showed less variability than the collagen binding-based activity assay. Antigen values correlated well with activity in normal individuals, but were discrepant to various degrees in neonates, pregnancies of later maternal age and cardiac surgery. No discrepancies were noted in liver cirrhosis and inflammatory bowel disease, which were both associated with low-plasma levels of ADAMTS13. The parallel measurement of ADAMTS13 activity and antigen provides a new tool for understanding the behaviour of the VWF cleaving protease in health and disease.