Infusions and decoctions of the leaves, roots and inflorescences of the herbaceous shrub Chenopodium ambrosioides (American wormseed, goosefoot, epazote, paico) and related species indigenous to the New World have been used for centuries as dietary condiments and as traditional anthelmintics by native peoples for the treatment of intestinal worms. Commercial preparations of oil of chenopodium and its active constituent, ascaridol, obtained by steam distillation, have been and continue to be, used with considerable success in mass treatment campaigns. Ethnopharmacological studies in a community of Mayan subsistence farmers in Chiapas, Mexico, confirmed that decoctions containing up to 300 mg of dry plant material per kg body weight (MGKGW) were widely used and traditionally highly regarded in the treatment of ascariasis. However, therapeutic doses of up to 6000 MGKGW of powdered, dried plant had no significant anthelmintic effect on the adults of Necator, Trichuris of Ascaris. Gas-liquid chromatographic analyses of plant samples used consistently demonstrated the presence of ascaridol in the expected amounts. Possible origins of subjective belief in the efficacy of C. ambrosioides as used, may be related to the positive association of spontaneous, or peristalsis-induced passage of senescent worms immediately following a therapeutic episode. It is also possible that in the past varieties of the plant containing much more ascaridol were used. The results of these controlled field studies did not sustain widely held traditional beliefs, nor the value of therapeutic practices regarding this plant. It is, therefore, essential that all indigenous ethnomedical practices be objectively evaluated for efficacy and safety using appropriate protocols before being considered for adoptation or promotion in health care programs.(ABSTRACT TRUNCATED AT 250 WORDS)