The severity and variability of platelet dysfunction in preoperative arthritic patients taking nonsteroidal anti-inflammatory drugs (NSAIDs) have not been well studied previously. We evaluate 30 preoperative patients taking diclofenac (group D) by routine coagulation screen, platelet count, fibrinogen concentration, thrombelastography, and PFA-100 (Dade Behring, Inc, Deerfield, IL)) platelet function analyzer. Ten patients (group P) and 30 healthy volunteers (group N) not taking NSAIDs serve as control. Diclofenac causes significant prolongation of mean PFA-100 closure times (P < .0001). However, the prolongation is highly variable; and up to 33% of patients are still having normal platelet function despite diclofenac consumption. Low body weight is a significant predictor of more severe platelet dysfunction (P < .01). Other tests are not useful. We conclude that not all patients taking NSAIDs have similar platelet dysfunction and that preoperative monitoring with PFA-100 is preferable.