Intraoperative bleeding due to platelet disorders is a persistent problem. Therefore, a screening assay to identify patients who are likely to bleed as a result of platelet dysfunction would be useful in formulating decisions about patient care. A previous study indicated that preoperative collagen-induced whole blood platelet aggregation predicts bleeding in patients undergoing surgery with cardiopulmonary bypass, a procedure associated with substantial blood loss. In the current study, we assessed the ability of the same whole blood platelet aggregation test to predict blood loss in patients undergoing surgical procedures not associated with substantial blood loss. The study included 369 adult patients (165 men and 204 women). Patients were categorized in three groups depending on the invasiveness of the operation and the expected blood loss. The intraoperative estimated blood loss value, obtained from the operative report in the patient record, increased significantly with increasing surgical invasiveness. Patients with excessive blood loss (defined as blood loss at or above the 75th or 90th percentile of the estimated blood loss values of patients undergoing procedures of similar invasiveness) had similar platelet aggregation values as patients who did not experience excessive blood loss. Thus, for patients undergoing operations not associated with substantial blood loss, the results of preoperative collagen-induced whole blood platelet aggregation are not effective in identifying patients likely to experience excessive blood loss.