Metronidazole pharmacokinetics were studied in patients with different degrees of liver cirrhosis, classified according to the Child‐Pugh algorithm (A, B or C, as liver disease severity increases) and in schistosomic patients. Metronidazole (500 mg) was administered i.v. as a slow infusion over 20 min, and blood samples were collected at set intervals after the end of the infusion. The plasma concentrations of metronidazole and its main metabolite hydroxy‐metronidazole were quantified by reversed‐phase h.p.l.c. with u.v. detection. The metronidazole and hydroxy‐metronidazole areas under the curve from 0 to 24 h (AUC0,24h), the metronidazole terminal elimination half‐life (t1/2), the total clearance (CL), the metronidazole volume of distribution (V) values and the hydroxy‐metronidazole/metronidazole concentration ratios as a function of time were calculated for each group. Comparison of the metronidazole AUC0,24h, t1/2 and CL values revealed that metronidazole metabolism is progressively impaired as the severity of liver disease increases. There were no variations in these parameters between the schistosomic and Child‐Pugh A groups. In addition, there were no differences in the V and hydroxy‐metronidazole AUC0,24h among the various groups studied. However, metronidazole metabolism was delayed in patients with hepatic disease, as illustrated by the hydroxy‐metronidazole/metronidazole ratio 10 min after the end of metronidazole infusion. These results indicate that the clinical assessment of liver disease is paralleled by an impairment of metronidazole metabolism. Of the studied variables, we propose the hydroxy‐metronidazole/metronidazole ratio 10 min after metronidazole infusion as a suitable and practical index for liver function evaluation.