To determine whether the aminopyrine breath test (ABT) and serum bile acid concentrations reflect histologic severity of chronic hepatitis, 56 patients were prospectively evaluated with liver biopsies and ABTs, and the results were compared to results of standard liver chemistry tests; 44 of these patients also had fasting and 2-hr postprandial serum bile acid measurements. Mean values for the ABT were significantly lower, and serum bile acids were significantly higher in patients with chronic active hepatitis with bridging or cirrhosis than in patients with mild chronic active hepatitis or chronic persistent hepatitis. Thirty of 35 patients with bridging or cirrhosis had ABTs <5.7% (mean of controls –2 S.D.) and 21 of 25 with mild chronic active hepatitis or chronic persistent hepatitis had values >5.7% (sensitivity 0.86, specificity 0.84). Fasting and 2–hr postprandial bile acids were also more sensitive than standard chemistries in identifying patients with bridging or cirrhosis. Thus, the ABT and serum bile acids reflect histologic severity in chronic hepatitis patients and may be helpful in selecting patients for liver biopsy.