A high incidence of infection with the threadworm Enterobius vermicularis has been demonstrated in a group of children with allergic asthma, but also in a non-allergic group. There was no evidence of infection with any other helminth parasites. Presence or absence of threadworm infection was not clearly correlated with differences in total serum IgE level in either allergic or non-allergic children although levels of this immunoglobulin were raised in the former group. Some of the allergic children gave positive skin reactions following intradermal injection of an antigen extracted from E. vermicularis. The suggestion is therefore put forward that hypersensitivity to E. vermicularis allergen absorbed from the bowel might contribute to the allergic symptoms. The results are also discussed in relation to recently discovered experimental evidence that helminth infection may non-specifically potentiate unrelated IgE antibody responses.