Fifteen patients with cholestatic disorders were treated for 1 to 5 months with phenobarbital. Primary biliary cirrhosis was diagnosed in seven, sclerosing cholangitis in two, intrahepatic biliary hypoplasia in three, and cholestatic hepatitis in three. Except for the patients with cholestatic hepatitis, in whom marked cholestasis was virtually the only abnormality in liver biopsy specimens, serum bilirubin and bile acid concentrations were diminished during therapy, the hepatic clearance of sulfobromophthalein and 131-I-rose bengal was variably enhanced, and there was relief from pruritus. Serum cholesterol concentrations and other measures of hepatic function were not significantly changed during therapy except for serum alkaline phosphatase activity, which rose in twelve patients. Parallel changes occurred in 5'-nucleotidase, suggesting a hepatic origin for the alkaline phosphatase activity. These studies indicate that phenobarbital therapy is associated with improvement in organic anion clearance in some patients with cholestatic disorders and may be beneficial to such patients.