Background Congenital prekallikrein deficiency has been associated with increased risk of thrombosis in a few uncontrolled studies. Prekallikrein levels were reduced by 36-94% (median 75%) in a phase 2 study with the prekallikrein specific antisense oligonucleotide, donidalorsen, in patients with hereditary angioedema (HAE). Objectives To estimate the effects of plasma prekallikrein reduction on coagulation and fibrinolysis. Methods Plasma samples were obtained from 16 HAE patients treated with donidalorsen and six placebo-treated patients. Calibrated automated thrombogram (CAT), clot lysis time, high-molecular-weight kininogen activity, factor XI activity, prothrombin fragment 1+2 (F1+2), thrombin-antithrombin complexes, D-dimer, plasminogen activity, plasmin-α2-antiplasmin complexes, and α2-antiplasmin activity were measured before and after four months of treatment. Results No significant changes following donidalorsen treatment were observed between baseline and after four months of treatment: CAT lag time 3.0 min and 3.0 min, CAT peak thrombin concentration 109% and 129%, CAT endogenous thrombin potential 98% and 108%, factor XI activity 110% and 115%, F1+2 levels 251 pMol/L and 161 pMol/L, thrombin-antithrombin complexes 2.6 μg/L and 1.9 μg/L, high-molecular-weight kininogen activity 87% and 93%, clot lysis time 92% and 99%, D-dimer levels 0.65 μg/L and 0.36 μg/L, plasminogen activity 116% and 125%, plasmin-α2-antiplasmin complexes 533 ng/mL and 328 ng/mL, and α2-antiplasmin activity 122% and 127%, respectively. There were also no differences between donidalorsen and placebo treatment. Conclusions Reduction of plasma prekallikrein by donidalorsen in HAE patients neither affected thrombin formation nor fibrinolytic activity. Our data suggest that partial plasma prekallikrein reduction does not influence thrombotic risk.