Different methods of expressing the results of the aminopyrine C02 breath test (ABT) were compared to determine the method that would be most sensitive for evaluating liver function. The patient population included healthy controls (n = 22); patients with mild (4), moderate (9), and severe (7) alcoholic liver disease; and patients with chronic persistent hepatitis (7), chronic active hepatitis (18), chronic active hepatitis with bridging (18), and chronic active hepatitis with cirrhosis (17). The ABT was performed with 2 μCi [14C]aminopyrine or 2 mg per kg [13C]aminopyrine, and the results were expressed as the instantaneous labeled CO2 excretion rates at 30, 60, 90, and 120 min after the dose, the maximum excretion rate, and the 120-min cumulative excretion. The 30-min parameter had the highest sensitivity toward moderate alcoholic hepatitis, severe alcoholic hepatitis, chronic active hepatitis with bridging, and chronic active hepatitis with cirrhosis (96%); however, when the ABT was repeated in six normal controls with 225 ml of orange juice, the 30-min rate was significantly reduced (7.8 ± 1.3 vs. 5.6 ± 1.2% dose per hr), and the rate of false-positive test results was increased (0 of 6 vs. 2 of 6) indicating that gastric delay influenced the 30-min parameter. The 60-min excretion rate, maximum rate, and 120-min cumulative excretion were the next most sensitive expressions of the ABT, and were not altered by the small gastric load. Of these three expressions, the 60-min parameter was the most convenient because it required fewer samples and a shorter test period.