Praziquantel undergoes extensive first-pass hepatic biotransformation, but there is little information on its disposition or toxicity when administered to patients with liver disease. To define the influence of liver disease on the pharmacokinetics of praziquantel, we administered it orally to 30 patients with proven Schistosoma japonicum infection whose liver disease was carefully assessed as being severe, moderate, or absent. Both the peak plasma concentration of praziquantel and the bioavailability (measured as the area under the plasma concentration time curve) were significantly greater in the two groups of patients with liver disease (P less than .005), as were the concentrations of the two identified metabolites of praziquantel. Mild side effects were associated with high peak concentrations of praziquantel, but a syndrome of severe abdominal pain followed by bloody diarrhea was not. Our results indicate that the side effects and bioavailability of praziquantel are increased in the presence of liver disease.