Alteration of coagulation status and certain clinical chemistry laboratory determinations of 75 adult patients undergoing cardiopulmonary bypass procedures for acquired heart disease was studied during and after surgery. None of the patients was given transfusions of blood or blood components. With hemodilution, the mean hematocrit value dropped from 38% to 28% during the procedure. Fibrin degradation products and euglobulin lysis time were transiently abnormal. Factor V diminished somewhat during the procedure, whereas factors VIII and IX increased after surgery. Clottable fibrinogen values decreased slightly, but increased to an abnormally high value at 24 and 48 hours. Mean value of platelet counts decreased from 194,000 to 144,000/microliter immediately after surgery. Knowledge of expected deviation of coagulation factors and certain clinical chemistry tests following open heart surgery is helpful in evaluating the status of the postoperative patient.