Serum uric acid (UA) has been proposed as a marker for impaired oxidative metabolism and the present study investigated whether serum UA level increases in proportion to the severity of pulmonary thromboembolism (PTE) in 193 patients. Serum UA was repeatedly measured after treatment of PTE in 76 patients. Right heart catheterization was performed in a subgroup of patients (n=104). Serum UA on admission was significantly elevated in patients with acute PTE (6.2±2.3 mg/dl) and those with chronic PTE (7.0±2.1 mg/dl) compared with age-matched controls (4.5±0.9 mg/ml). In particular, serum UA was markedly higher in the 27 patients who died during hospitalization than in the remaining survivors (8.3 ±2.2 vs 6.5±2.2 mg/dl, p<0.001). In acute PTE, serum UA negatively correlated with cardiac output, but not significantly with mean pulmonary arterial pressure. In chronic PTE, serum UA negatively correlated with cardiac output and positively correlated with mean pulmonary arterial pressure. Serum UA significantly decreased from 6.7±2.0 to 5.8 ±1.9 mg/dl with treatment, associated with an increase in cardiac output and in PaO₂. Serum UA increases in proportion to the severity of PTE, and thereby may serve as a potential indicator of the efficacy of treatment of PTE. (Circ J 2002; 66: 571 - 575)