Thromboxane A2 (TXA2) is produced from arachidonic acid by cyclooxygenase-1 (COX-1) in thrombocytes and for the most part is converted to 11-dehydro-thromboxane B2 (11dTXB2), the final stable metabolite. Since daily excretion amounts of 11dTXB2 in urine are well correlated with the degree of inhibition of COX-1 by aspirin, inhibition is evaluated by measuring 11dTXB2 excretion and this is usually done through 24-hour urine collection. However, as this involves a lot of work, we examined the validity of using spot urine measurements for this purpose. The intra-day variation in urinary 11dTXB2 levels was studied in five patients who had been taking low-dose aspirin for an extended period. Urinary concentrations of 11dTXB2 were measured using a commercial enzyme immunoassay kit and the urinary excretion rate of 11dTXB2 was expressed as the ratio of the concentration of 11dTXB2 to that of creatinine. The results showed that there was no marked variation in intra-day urinary excretion rates and that the range of variation from 24-hour urine measurements was no more than about 15%. They therefore indicate that spot urine could be used in place 24-hour urine collection for measuring 11dTXB2 levels.