The incidence of childhood Type I (insulin-dependent) diabetes mellitus has risen in parallel with that of childhood asthma, and the hygiene hypothesis proposes that this is due to reduced stimulation of the immune system by early intercurrent infection. If so, this protective effect is probably mediated by regulatory T lymphocytes. Co-evolutionary partners might have contributed to the development of this form of response, and parasites and the indigenous biota of the gut are plausible candidates. Helminths inhibit the development of atopic disease via induction of regulatory T cells and secretion of Il-10, and pinworms inhibit diabetes development in the non-obese diabetic (NOD) mouse. The most successful human helminth of the western world is the pinworm Enterobius vermicularis, and some 50% of young children in Europe and North America may have been infested around the middle of the twentieth century. Pinworms are benign, usually asymptomatic, and may have immunomodulatory properties that protect against the development of immune-mediated disorders including diabetes and asthma. Their decline in response to improved living conditions might explain a number of features of the epidemiology of childhood atopy and diabetes. The proposed role would be one of immunomodulation rather than disease induction, possibly mediated by interaction with other influences upon the development of the mucosal immune system. This hypothesis could be tested in case-control studies by the development of serological markers or skin testing. If confirmed, identification of the underlying mechanisms could open the way to new forms of immune intervention.