BACKGROUND AND AIM: Recent studies suggest the development of a procoagulant state with hemodilution. We conducted this study to investigate the effect of hemodilution, by the priming solution in a cardiopulmonary bypass (CPB) circuit, on "point of care" coagulation assays (activated clotting time [ACT] and thromboelastography [TEG]). METHODS: Twenty patients undergoing cardiac surgery with crystalloid priming of CPB circuit were evaluated. Confounding variables arising from contact activation were eliminated by minor modifications. Ten milliliter per kilogram body weight of priming solution (lactated Ringer's) was infused via the aortic cannula. ACT and TEG were performed, both prior to and immediately after hemodilution. In case of latter, four variables, reaction time (r), coagulation time (k), maximum amplitude (MA), and clot formation rate (angle alpha), were estimated and considered for the results. To see if these results are duplicated "in vitro," prebypass blood samples from eight heparinized patients, diluted (4:1) with priming solution from the venous reservoir, were also analyzed. RESULTS: Falls in ACT, from a mean of 659.7 (+/-260.6) seconds to 251.5 (+/-103.2) seconds (p < 0.01), r time (678.1 [+/-318.1] sec to 468.7 [+/-152.7] sec) (p < 0.01), and k time (211.7 [+/-161.7] sec to 123.8 [+/-32.1] sec) (p < 0.05) on TEG were noted upon hemodilution. Angle alpha and MA increased, but were not statistically significant. Results from the in vitro study closely matched the results from our in vivo analysis. CONCLUSION: The study suggests that hemodilution by crystalloid priming solution may impair the efficacy of anticoagulation during CPB. The mechanism for this phenomenon remains to be elucidated.