For decades diet, particularly high habitual sodium intake, has been implicated in the etiopathogenesis of high blood pressure. This concept has been based on clinical observations, including therapeutics and results of dietary trials; on animal experimentation, particularly induction of hypertension by salt feeding; and on epidemiologic findings. While considerable knowledge has been amassed in support of this concept, limitations and inconsistencies exist in the data base. This is especially true in regard to the findings from population studies. Thus, negative or inconsistent results have been reported from several cross-sectional studies of dietary sodium and blood pressure in which the unit of measurement has been the individual within a population. When INTERSALT began in 1984, no prospective data were available from population-based longitudinal investigations. A major problem in such studies, one possibly accounting for the negative and inconsistent findings, was the difficulty of validly measuring the habitual sodium intake of individuals within a population in which there was relative homogeneity in regard to this trait. This difficulty was due to large individual variation day-to-day in the sodium intake and output in most populations, so that the ratio of intra- to inter-individual variances was large. This led to high probability of misclassification and obscuring or weakening of true associations in any single population sample, unless multiple measurements were made for each person. Since dietary history methods such as 7-day food records, multiple 24-hour recalls, and dietary interviews were of limited validity for assessment of sodium intake, investigators had to rely on timed urine collections, especially 24-hour samples. But accurate collection of several such specimens per person by many individuals for population studies was a difficult undertaking. This was an important reason for the paucity of valid data on sodium and blood pressure from within-population studies.