Rationale: Observational studies and initial randomized trials have indicated that a low sodium diet may improve asthma control. Objectives: We tested the hypothesis that a low sodium diet would improve asthma control over a six week period. Methods: Participants with a physician diagnosis of asthma and measurable bronchial reactivity to methacholine entered a randomized double-blind placebo-controlled trial. All adopted a low sodium diet and were randomized to receive either 80 mmol/day of oral sodium supplements (normal sodium intake) or matched placebo (low sodium intake) for six weeks. The primary outcome was change in bronchial reactivity to methacholine; secondary outcomes were change in lung function, allergen skin prick test responses, morning and evening peak expiratory flow, asthma symptoms score, daily bronchodilator use, Juniper Standardised Asthma Quality of Life Questionnaire score, and atopy. Main measurements and results: 220 individuals entered the study, of whom 199 completed the protocol. In the low sodium intake group, mean daily urinary sodium excretion decreased by 20 mmol (standard deviation; SD: 64 mmol) and in the normal sodium intake group increased by 28 mmol (SD: 74 mmol). There were no differences between the two groups in the primary or secondary outcome measures; the mean difference in bronchial reactivity between the low and normal intake groups being -0.03 doubling doses of methacholine (95% Confidence Intervals -0.60 to 0.53). Conclusions: The use of a low sodium diet as an adjunctive therapy to normal treatment has no additional therapeutic benefit in adults with asthma and bronchial reactivity to methacholine. Clinical Trial Registry Information: ID# ISRCTN80771653 registered at www.controlled-trials.com