Methodology of sodium sensitivity assessment. The example of age and sex. This article addresses the methodology of sodium sensitivity assessment. There have been reports to suggest that a high sodium intake is a cause of elevated blood pressure and trials to indicate that a reduction in sodium intake may reduce blood pressure that is already high; the implications of these findings are discussed. Many studies on sodium sensitivity suffer from what could be called the "normal probability fallacy"; without appropriate control conditions, an intervention such as sodium restriction may incorrectly be assigned to a more pronounced response in a subgroup of the study population. As an example, findings are reviewed of age and sex as determinants of a response of blood pressure to variations in sodium intake. The limited data available suggest that subjects that are older and have higher blood pressure levels seem to benefit more from a reduction in sodium intake. In addition, elderly subjects at a high dietary sodium intake may have a higher risk of developing hypertension. Findings in both nonexperimental and experimental studies tend to support this view. Findings on sex differences are less consistent. A systematic approach to the assessment of factors and mechanisms responsible for sodium sensitivity in some subjects is needed to determine who might benefit most. Until more data are available, there is little basis to discriminate sodium-sensitive from sodium-resistant hypertensive subjects. 