The behavior of urinary thromboxane B2 (TXB2) during acute rejection of lung allotransplants was evaluated. Unmatched mongrel dogs were submitted to a left lung orthotopic allotransplantation (groups I and II), or a sham operation (group III). All animals had an initial significant elevation of TXB2 excretion due to surgical trauma; however, in sham-operated animals (group III) this elevation returned to basal levels after 3 days. All transplanted animals (groups I and II) had persistent TXB2 elevation with 2 important peaks on postop days 5 and 9. The elevated TXB2 excretion persisted in spite of immunosuppressive treatment with azathioprine and prednisone (group II). Rejection was followed by means of an objective grading system applied to chest roentgenograms taken on all animals. It was found that TXB2 levels correlated directly with the grade of radiographic changes seen, thus indicating degree of rejection. TXB2 can be useful as a noninvasive indicator for surveillance of lung allograft rejection.