From 1996 to 2003, four 12-month population-based surveys were performed in FoodNet sites to determine the burden of diarrhoeal disease in the population. Acute diarrhoeal illness (ADI) was defined as [ges ]3 loose stools in 24 hours with impairment of daily activities or duration of diarrhoea >1 day. A total of 52840 interviews were completed. The overall weighted prevalence of ADI in the previous month was 5·1% (95% CI±0·3%), corresponding to 0·6 episodes of ADI per person per year. The average monthly prevalence of ADI was similar in each of the four survey cycles (range 4·5–5·2%). Rates of ADI were highest in those age <5 years. Of those with ADI, 33·8% (95% CI±2·7%) reported vomiting, 19·5% (95% CI±2·1%) visited a medical provider, and 7·8% (95% CI±1·4%) took antibiotics. Rates of ADI were remarkably consistent over time, and demonstrate the substantial burden placed on the health-care system.