The estimation of disease frequency using a population sample. International Journal of Epidemiology, 1978, 7: 277--284. A method of determining disease frequency is developed. The statistical theory allows the estimation of incidence or prevalence credibility intervals both for the collection of all cases and where resources are such that only a representative sample of all cases is used. The sampling technique may be of value in areas where financial and manpower resources are limited. In most situations a single, professional investigator could produce satisfactory results.