Fasting serum bile acid levels were measured by gas-liquid chromatography in 56 patients with primary biliary cirrhosis. Of these, 52 (93%) had increased levels (greater than 2mug/ml), including 14 of the 18 with normal serum bilirubin concentrations. The four patients with normal bile acid levels had early lesions as judged by histological and clinical criteria. With progression of the disease, as indicated by the histological features of the lesions, total bile acid levels increased, and the ratio of serum cholic-to-chenodeoxycholic acid decreased. Ratios of serum cholic-to-chenodeoxycholic acid below 1 occurred predominantly in patients with advanced or terminal disease. These studies suggest that serial measurement of serum bile acids may aid in the evaluation of primary biliary cirrhosis.