To clarify the role of the serum urate level and its change as a potential marker for severe tissue hypoxia, we have measured serum urate levels and urine uric acid excretion in 16 patients with acute cardiovascular disease. The six patients who died had a baseline mean serum urate level of 11.1 mg/lg (range, 6.6 to 15.5 mg/dl) and reached a peak mean value of 20.7 mg/dl (range, 13.6 to 33.0 mg/dl). Five of these patients had findings to suggest increased production of uric acid, in addition to decreased excretion of uric acid from impaired renal function. The 10 survivors had a baseline mean serum urate level of 6.8 mg/dl (range, 1.3 to 14.0 mg/dl) and a maximal mean peak value of 7.1 mg/dl (range, 2.9 to 14.0 mg/dl). There was no consistent evidence for increased production or decreased excretion of uric acid. Patients who died had a lower systolic blood pressure, arterial pH and plasma bicarbonate level and a higher heart rate and serum creatinine level compared with the patients ho survived. The observations suggest that marked hyperuricemia at the height of an illness may predict a fatal outcome. Tissue hypoxia may contribute to this sequence of events by leading to the depletion of adenosine triphosphate (ATP) and activation of purine nucleotide degradation to uric acid.