A double-masked study was conducted to determine the efficacy and safety of randomly allocated chenodiol (chenodeoxycholic acid, 750 mg/d or 350 mg/d) or placebo administered for 2 years to 916 patients for dissolution of radiolucent gallstones. There was confirmed complete dissolution in 13.5% of patients (750 mg/d), 5.2% (375 mg/d), and 0.8% (placebo), p less than 0.0001. Partial (over 50%) or complete dissolution (by validated roentgenographic metrology) occurred in 40.8% (750 mg/d), 23.6% (375 mg/d), and 11.0% (placebo), p less than 0.0001. Dissolution occurred more frequently in women, thin patients, or patients with small or floating gallstones or serum cholesterol greater than or equal to 227 mg/dL. Clinically significant hepatotoxicity occurred in 3% of patients (750 mg/d), 0.4% (375 mg/d), and 0.4% (placebo), p less than 0.007, and always was reversible biochemically. Elevations of 10% or more of serum cholesterol, mostly low-density lipoproteins, occurred in 85.2% of patients (750 mg/d), 82.8% (375 mg/d), and 67.0% (placebo), p less than 0.001. Chenodiol, 750 mg/d for up to 2 years, is appropriate therapy for dissolution of gallstones in selected patients who are informed of the risks and benefits.