The construction of a composite effectiveness measure was explored using clinical data collected routinely in trials of drug therapies for asthma. The measure is the episode-free day (EFD), where an ‘episode’ is either an asthma attack, the need for rescue medication, sleep disturbance caused by asthma, or an adverse event. The EFD measure was used in a retrospective cost-effectiveness analysis of a previous Phase III controlled clinical trial of formoterol versus salbutamol, in which 145 patients with bronchial asthma were randomised to receive maintenance therapy with either inhaled formoterol or inhaled salbutamol over a 12-week period. Average and incremental cost-effectiveness ratios were assessed for the 2 drugs in terms of the total expected cost of drug plus rescue therapy, and EFD rates. The analysis suggests that, with relatively little addition to clinical data collection, economically and clinically meaningful composite measures can be constructed to assist in making cost-effectiveness comparisons between alternative asthma therapies.