INTERSALT is a collaborative study of the relationship between blood pressure and sodium and potassium intake in more than 50 population samples from 32 countries, representing a wide range of sodium intakes. The study uses standardized techniques for blood pressure measurement, collection of 24-h and spot urine samples, and recording the major confounding variables. Investigators, who have been trained at regional meetings, follow rigorous quality control checks. Laboratory work and data analysis are all conducted centrally. This report presents the study design and methods.