A review of campylobacter infection in Scotland over five years (1978-82), during which 7808 human isolates were recorded (mean annual isolation rate of 30 per 100000), revealed differences in the epidemiology of the disease between rural and urban populations which were not apparent in the national data. The incidence of infection in the two rural areas studied was greatest in the early months of the year, whereas that in the two urban areas showed a third-quarter predominance. In both urban and rural populations, age-specific infection rates were highest in children less than 5 years old, but this trend was more pronounced in rural than urban populations. Conversely, secondary peaks in age-specific infection rates observed in young adults were more pronounced in the urban than rural populations. It is postulated that rural children were being infected by campylobacters at an early age by drinking contaminated raw milk which was not normally available to city residents. The lower incidence in adults in the rural populations is interpreted as indicating more widespread immunity, resulting from greater exposure to infection during childhood. The effect of compulsory heat treatment of milk sold in Scotland, introduced in August 1983, is currently being studied.