Most prevalence surveys (figures 1 and 2) suggest that asthma is less common in heavily parasitized countries, but case-control studies either show no association or an increase in parasitism in asthmatics. Studies which included egg counts suggest that asthmatics have a lower parasite burden than normals, compatible with the notion that asthma protects against parasitic infection, or vice versa. The completely contradictory findings of serum IgE in three studies do not help to elucidate any association or its mechanism. The available epidemiologic data neither refute nor support the theory that parasitic disease protects against or causes asthma. A more definitive answer might come from a comparison of asthma prevalence in heavily parasitized and parasite-free subjects, or from a prospective study of asthma incidence in two or more comparable communities where one population is naturally or therapeutically free of intestinal parasites. If an effective antihelminthic vaccine is developed, it would also be interesting to watch vaccinees for development of, or changes in, asthma symptoms, and to determine whether these findings correlate with vaccine-induced changes in IgE.