Differential inhibition of thromboxane A2 (TxA2) and prostacyclin (PGI2) biosynthesis has an antithrombotic potential, since it may change the TxA2/PGI2 formation ratio in a favourable direction. Very low doses of acetylsalicylic acid (ASA) have been demonstrated to elicit differential inhibition of TxA2 and PGI2 formation in healthy subjects; whether a similar effect can be obtained in patients with atherosclerosis is still an open question. We addressed this by analyzing the urinary excretion of the 2,3-dinor-metabolites of TxA2 (Tx-M) and PGI2 (PGI-M) in 10 patients with severe atherosclerosis during 10 consecutive days. The first three days were a basal period, under which no treatment was given. During the subsequent seven days a daily 50 mg oral dose of ASA was administered. In the basal state urinary Tx-M did not differ from that of PGI-M, the median excretion rates of the two eicosanoid metabolites being 526 (range 68-1490) and 562 (range 93-1970) pg/mg creatinine, respectively. During ASA treatment urinary Tx-M fell to a lower (p less than 0.001) level than PGI-M. Thus, during the last 5 days of ASA treatment the median excretion of Tx-M was depressed (p less than 0.001) to 148 (range 48-428) pg/mg creatinine, while that of PGI-M was decreased (p less than 0.01) to 313 (range 42-2658) pg/mg creatinine. These data indicate that a daily 50 mg dose of ASA inhibits cardiovascular formation of eicosanoids in patients with severe atherosclerosis and increased platelet TxA2 formation. Furthermore, this dose of ASA inhibits the formation of TxA2 more than that of PGI2.