Background: Unlike salmonellosis with well-known routes of transmission, the epidemiology of campylobacteriosis is still largely unclear. Known risk factors such as ingestion of contaminated food and water, direct contact with infected animals and outdoor swimming could at most only explain half the recorded cases. Discussion: We put forward the hypothesis that flies play a more important role in the transmission of the bacteria, than has previously been recognized. Factors supporting this hypothesis are: 1) the low infective dose of Campylobacter, 2) the ability of flies to function as mechanical vectors, 3) a ubiquitous presence of the bacteria in the environment, 4) a seasonality of the disease with summer peaks in temperate regions and a more evenly distribution over the year in the tropics, 5) an age pattern for campylobacteriosis in western travellers to the tropics suggesting other routes of transmission than food or water, and finally 6) very few family clusters. Summary: All the evidence in favour of the fly hypothesis is circumstantial and there may be alternative explanations to each of the findings supporting the hypothesis. However, in the absence of alternative explanations that could give better clues to the evasive epidemiology of Campylobacter infection, we believe it would be unwise to rule out flies as important mechanical vectors also of this disease. © 2005 Ekdahl et al, licensee BioMed Central Ltd.