A relation between the prevalence of asthma and economic development has been suggested by studies in migrants and other surveys in developing countries. That this correlation might be partially explained by an increased intake of salt in the diet is supported by the observation that sales of table salt in the different regions of England and Wales are independently correlated with mortality from asthma for men and for children. As part of a wider survey of asthma 138 men living in two Hampshire villages and aged 18-64 were given a bronchial histamine challenge test and had their 24 hour urinary excretion of sodium measured. Bronchial reactivity was strongly related to 24 hour excretion of sodium after allowing for the effects of age, atopy, and cigarette smoking, there being on average a 10-fold difference in reactivity over the 95% range of sodium excretion recorded in the study. The data suggest that a high sodium diet may potentiate bronchial reactivity.