The immunoepidemiological interactions between intestinal worm (or geohelminth) infections and allergy are of great interest to parasitologists, immunologists, and allergists because of the close similarities between the human immune response to geohelminth parasites and environmental allergens. Allergic diseases appear to be most rare in populations living in the rural tropics with high rates of infection with geohelminth parasites, and this has led to suggestions that the relationship between geohelminth infections and allergy may be causal. Allergic sensitization and disease results from a complex interaction between environmental exposures and genetic background, and the numerous epidemiological studies that have investigated the relationship between allergy and geohelminth infections have provided conflicting findings. The strongest epidemiological evidence for a causal association is provided by intervention studies that demonstrate evidence for an effect of anthelmintic treatment on atopy or asthma risk. There is evidence also for an inverse relationship between geohelminth infection and either atopy or asthma symptoms from cross-sectional studies that have been conducted in areas of high infection prevalence. Chronic geohelminth infections could affect allergy risk by modulation of the immune response to environmental allergens, and an area of great research activity at present is the investigation of the role of regulatory T cells in modulating host inflammatory responses. However, a causal association between geohelminth infections and allergy remains to be proven, and prospective and intervention studies are required that investigate the development of allergy in early life at a time when humans are first exposed to geohelminth parasites and their antigens.