In contrast to long-term trials in other diseases which often focus on life-threatening events in evaluating treatments, long-term trials in asthma focus on treatments that aim to promote quality of life by reducing the symptomatic impact of the disease. This gives rise to different issues in the design of these trials, some of which are discussed in this paper. The choice of primary endpoint and whether measures of symptomatic state should be preferred to laboratory measures of lung function are considered. Whichever is adopted, summary measures of repeated responses provide results which are easily interpretable and help to focus design and analysis on the specific objectives of long-term studies. In particular, trials should aim to establish that a treatment difference exists after a prolonged period of treatment and that trends in response predict that this difference will be maintained. Consideration is given to how these objectives can be met more efficiently by appropriate timing of assessments, and also to the value of interim analyses and the impact of patient withdrawals.
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