There have been several reports on alterations of platelet function and raised plasma heparin levels in symptom-free atopic subjects. Either of these can affect formation of thrombin in vivo. In 25 symptom-free atopic patients and 32 healthy volunteers we studied the generation of thrombin in blood emerging from a standardized skin microvasculature injury, which also served to determine bleeding time. Generation of thrombin was delayed in atopies. They produced significantly less thrombin (P<0.01) during the early and central phase of haemostasis. The amount of thrombin generated was inversely correlated to bleeding time, which in atopies was on average 50 sec longer than in controls (P= 0.055). Two hours after ingestion of 500 mg aspirin, this difference increased up to 150 sec, although the individual responses varied markedly (P= 0.08), while the generation of thrombin became strongly depressed in both groups. The possible clinical relevance of the delayed formation of thrombin in atopy awaits further studies.