To evaluate in vivo platelet activation, 11-dehydro-thromboxane B2 levels in plasma and urine were measured in 9 patients with unstable angina and 11 with stable angina using radioimmunoassay modified by the extraction method of Kawano et al. The 2 groups were matched for age, sex, coronary risk factors, medications or atherosclerotic lesions in coronary angiography. Although there was no difference in the plasma level between the 2 groups in the usual state, urinary 11-dehydro-thromboxane B2 amount in unstable angina was significantly increased compared to the stable angina group (865.5 +/- 238.7 vs 535.9 +/- 177.4 pg/mg creatinine (mean +/- SD), p < 0.01). There was no correlation between the 11-dehydro-thromboxane B2 level and the degree of coronary atherosclerosis in either group. The plasma level increased during the attacks in 2 patients with unstable angina. The amount of urinary 6-keto-PGF1 alpha did not differ between the 2 groups. These findings suggest that platelet activation in vivo is more pronounced in unstable angina than in stable angina, and that the measurement of urinary 11-dehydro-thromboxane B2 may be useful for evaluating and treating angina.