A common mutation in the prothrombin gene (G20210A) is associated with elevated prothrombin levels and thrombosis. The pathomechanism related to the G20210A mutation is currently not understood and the interdependence of prothrombin activity and prothrombin concentration in plasma is still poorly defined. Six of 191 blood donors examined in the present study carried the prothrombin allele G20210A. Despite the small number of cases, plasma samples from these individuals had significantly higher prothrombin activities than wild type carriers, whereas their prothrombin concentrations-although elevated-did not differ significantly from wild type. In subjects with the G20210A mutation there was also no significant correlation between prothrombin activity and concentration. Analyzing data from healthy blood donors without the prothrombin G20210A mutation, we found only weak correlations between prothrombin activity and concentration of immunoreactive prothrombin. Samples with a relatively high prothrombin concentration but low activity were observed as well as samples with a relatively high activity for a given concentration (hyperactive prothrombin). F1+2 concentrations as indicators of activated coagulation were only elevated in 13 of 125 investigated samples and could not explain any of these findings. Dysfunctional variants of prothrombin, a well known phenomenon, may be responsible for the former, and we speculate that posttranscriptionally modified prothrombin species may explain the observed functional diversity of this factor including hyperactivity. The genotype-phenotype association of the non-coding G20210A mutation is not clear cut. Therefore, further studies are needed to determine which factors apart from the known G20210A polymorphism regulate prothrombin concentration and/or activity and may trigger the manifestation of thrombosis.