This study investigates relationships between platelet reactivity and coronary blood flow obstruction in stable angina pectoris. Consented were 36 patients with single-vessel disease. The subjects were divided into two groups. One group (n=14) had less severe (<=80%) and the second group (n=22) had severe coronary flow impairment (90%). Before elective coronary angiography platelet in vitro reactivity in venous whole blood was determined using a flow cytometry technique. A thrombin-receptor activating peptide (TRAP-6) (0.77 and 0.06 g/l) and ADP (8.5 and 1.7 micromol/l) were used to activate platelets. The number of fibrinogen positive cells (%) i.e., activated platelets after stimulation was employed as experimental parameter. Less severe flow obstruction was associated with more reactive platelets. When stimulating with 0.77 g/l TRAP-6 the number of activated platelets was 64+/-15 (SD)%. The corresponding value for the group with severe flow obstruction was 40+/-17(SD)%. The difference is significant (P<0.001). 0.06 g/l TRAP-6 yielded similar results (P<0.01). Also when using 8.5 micromol/l ADP to challenge platelets less severe flow obstruction was associated with enhanced reactivity (P<0.01). 1.7 micromol/l ADP generated comparable results (P<0.05). Thus, in stable angina pectoris coronary flow obstruction is inversely related to platelet reactivity.