Studies of salt intake in hypertension. What can epidemiology teach us? It has been suggested that small changes in population blood pressure will have a major impact upon the incidence of cardiovascular disease caused by blood pressure elevation. Early reports indicated a close correlation between intercultural differences in salt intake and blood pressure. Before such epidemiological associations can be translated into population advice, certain conditions have to be met. The association has to be validated scientifically, and persuasive evidence has to be produced that the relationship is causal and reversible by changes in salt intake. Further, risk-benefit analysis should indicate that net harm is unlikely. Finally it has to be demonstrated that the population measures being advocated will produce an adequate change in dietary salt intake. The small individual effects upon blood pressure being examined, and the prevailing changes in blood pressure and cardiovascular mortality suggest that data will always fall short of the ideal and therefore that extrapolation will always be necessary. Nevertheless, a review of the present evidence indicates the inadequacies of the available data as a basis for population advice. 