Although Campylobacter jejuni is a frequent enteropathogen in cases of paediatric diarrhoea in developing countries, its route of transmission is not well understood. An age-matched, case-control study of children with C. jejuni diarrhoea was therefore carried out in Lima, Peru, from January 1983 to April 1986 to identify the risk factors and vehicles of transmissions. As cases, 104 children less than 3 years of age were selected and compared with controls of the same age with non-gastrointestinal illnesses. Household exposure to live chickens was an important risk factor (odds ratio, 11; after adjusting for socioeconomic and environmental variables). Subjects in index households had a higher frequency of infection than those in control households, and infected young children were more likely to be ill than older children or adults, suggesting that immunity may be acquired from natural infection. The risk factors identified suggest that direct contact with the faeces of C. jejuni-infected chickens in the household environment was largely responsible for transmission of the organism to susceptible infants.