In a double blind, randomised trial, the effects of 1 g aspirin and 1 g paracetamol were compared on bleeding time and platelet aggregation in 40 volunteers (20 females). Also investigated was the relationship between plasma aspirin esterase activity and both bleeding time and platelet aggregation after aspirin. Following 1 g aspirin there was a significant increase in bleeding time at 24 h (p less than 0.01). A significant reduction (P less than 0.01) in platelet aggregation with collagen was observed at 1, 6 and 24 h after aspirin, but no significant reduction (P greater than 0.05) was observed with ADP. Paracetamol had no effect on bleeding time or platelet aggregation. Plasma aspirin esterase activity ranged from 0.26-0.6 mumol/ml/min. A significant negative correlation (R = -0.55, P less than 0.001) was observed between percentage increase in bleeding time (24 h) and plasma aspirin esterase activity. Further significant correlations were observed between plasma aspirin esterase activity and change in platelet aggregation with collagen at 1 h (R = 0.68, P less than 0.001), 6 h (R = -0.73, P less than 0.001) and 24 h (R = -0.67, P less than 0.001). These results suggest that it might be possible to predict an individual's haemostatic response to aspirin from knowledge of their plasma aspirin esterase activity.