Background. We hypothesize that preoperative functional platelet number (platelet count multiplied by platelet aggregation percentage) are associated with 30-day mortality after cardiac surgery. Methods. We linked our preoperative testing database with the STS (Society of Thoracic Surgeon) database to form a study cohort of 1390 patients who had cardiac surgeries between January 2008 and December 2013. Preoperative tests of platelet count and platelet aggregation were routinely performed on all cardiac surgical patients within 24 hours before entering the operating room. Multiple logistic regression models were used to determine whether functional platelet number are associated with 30-day mortality, modified composite major adverse cardiocerebral events, postoperative renal failure or requirement for new renal replacement therapy, and reoperation for bleeding. Log-linear models were used to examine whether functional platelet numbers are associated with hospital length of stay and intensive care unit length of stay. Results. Functional platelet number had an inverse association with 30-day mortality, and each 50 × 109/L increase in functional platelet number resulted in decreased 30-day mortality (odds ratio of 0.767 with 95% confidence interval = 0.591-0.996). For secondary outcomes, functional platelet number was neither associated with major adverse cardiocerebral event nor length of stay. However, we found that each 50 × 109/L increase in functional platelet number was associated with decreased reoperations for bleeding (odds ratio of 0.778 with 95% confidence interval = 0.636-0.951). Conclusions. The preoperative functional platelet number had significant associations with 30-day mortality after cardiac surgery. Functional platelet number could be used to guide timing of cardiac surgery, especially as more and more patients are receiving antiplatelet medications nowadays.