A number of primary epidemiological characteristics are recognized as common to members of a syndrome designated "acute undifferentiated diarrhoeal disease". This syndrome includes both specific and non-specific diarrhoeal disorders. Within the existing knowledge and with the facilities available in less developed countries, an epidemiological basis for control, directed against the syndrome as a whole, is presented as the practical approach to community management. Clinical and microbiological distinctions do not extend to the main bulk of the problem. Individual epidemiological patterns exist according to age and varying social and ecological conditions. Field study by periodic home visits over four years has defined these patterns in highland rural villages in Guatemala. The chief problem was weanling diarrhoea.