The pharmacokinetics and therapeutic efficacy of praziquantel (Distocide®; Epico, El-Asher-Men-Ramadan City, Egypt) were studied in 40 patients with schistosomiasis mansoni and various degrees of hepatic dysfunction. The patients were allocated into four groups: the first included 10 patients with simple active schistosomiasis while the other three were made up of patients with schistosomiasis associated with liver cirrhosis and splenomegaly according to Child's classification of hepatocellular function. Every patient was treated with 40 mg/kg of praziquantel as a single oral dose. The efficacy of the drug was evaluated after two months by rectal snip examination. The pharmacokinetic parameters did not differ significantly between patients with simple active schistosomiasis (group 1) and those with hepatosplenomegaly with liver involvement but without ascites and jaundice (group 2). However, as liver cell dysfunction became more evident (groups 3 and 4), pharmacokinetic parameters of praziquantel such as the half-life of elimination, the half-life of absorption, the maximum concentration, the time to maximum concentration, and the area under the concentration-time curve increased proportional to the degree of hepatic insufficiency. Linear correlations were found between each of the these parameters on the one hand and hepatic function test results (total bilirubin, direct bilirubin, and serum albumin) on the other. In spite of these pharmacokinetic differences, the cure rates were 70%, 80%, 90%, and 90% in the four groups, respectively. Although the incidence of side effects was high (53%), such effects were transient and mild.