This study examines the exacerbating factors of hyperuricaemia in patients with cyanotic congenital heart disease (CCHD). We studied 59 CCHD patients aged 1 month-30 years. The following variables were assessed: serum uric acid levels, red blood cell count, haemoglobin, hematocrit, partial oxygen pressure and arterial oxygen saturation. Uric acid excretion and renal function were also measured in ten patients with serum levels of uric acid greater than 8 mg/dl (hyperuricaemia group). Serum uric acid level correlated significantly with age and severity of polycythaemia. However, it did not correlate with partial oxygen pressure or arterial oxygen saturation. Uric acid excretion was measured in hyperuricaemia group. Urinary uric acid excretion (24 h) was within normal limits in infants but markedly lower in patients over 15 years of age. The aetiology of hyperuricaemia and decreased uric acid fractional excretion and clearance in infants appears to be secondary to diminished excretion of uric acid in concert with uric acid overproduction. Hyperuricaemia in adolescents and adults with CCHD, however, results mainly from age-related impairment of uric acid excretion.