The plasma levels of mebendazole in children with hydatid cyst disease were measured with high pressure liquid chromatography. In 24 children who received mebendazole orally in a dose of 50 mg/kg, the mean (SD) level 4 hours later was 25.76(9.81) ng/ml (87.6(33) nanomole/l). This result was similar to those in most adult series. During more prolonged treatment, the plasma level 4 hours after the dose of the drug rose significantly with respect to the initial level (p < 0.05).