Treatment of soil-transmitted nematodes with mebendazole was carried out in 137 children aged 6--15 years in Thailand. There were 100 cases of hookworm infection, 37 of ascariasis, 16 of trichuriasis and 32 of strongyloidiasis. Mebendazole was given at 100 mg twice daily for three consecutive days irrespective of age or weight. The reduction rates as seen by mean egg counts four weeks after treatment were 94.9% for hookworm infection, 100% for ascariasis and 93.9% for trichuriasis; the reduction rates of the mean larval count in 15 cases of strongyloidiasis was 82.1%. No side-effects were observed. Mebendazole was thus confirmed as effective and safe in the treatment of soil-transmitted nematode infections in children in Thailand.