We studied the relationship between body mass index (BMI) on responses to asthma therapy using a retrospective analysis of four previously reported clinical trials. Fluticasone propionate (FP)/salmeterol via Diskus 100/50 microg twice daily and montelukast (MON) 10 mg daily were compared. BMI was classified as underweight (less than 20 kg/m(2)), normal (20-24.9 kg/m(2)), overweight (25-29.9 kg/m(2)), obese-1 (30-34.9 kg/m(2)), obese-2 (35-39.9 kg/m(2)), or obese-3 (at least 40 kg/m(2)). Outcomes assessed included forced expiratory volume in one second (FEV(1)), asthma symptom score, and albuterol use. FP/salmeterol produced greater improvements compared to MON in each of the asthma outcomes studied over the entire BMI range at the week-12 endpoint, with statistically significant differences noted among normal, overweight, obese-1, and obese-3 subjects. The within-treatment responses to FP/salmeterol across BMI ranges at the week-12 endpoint was statistically significantly greater in normal compared to obese-3 for FEV(1) and albuterol use, and in overweight compared to the obese-3 for each outcome studied. The within-treatment comparisons of MON across BMI ranges were significant for albuterol use in normal and underweight compared to obese-3 at the week-12 endpoint. Compared to subjects with normal BMI, the onset to peak FEV(1) may require longer treatment exposure in the very obese. Treatment responses to FP/salmeterol were consistently greater compared to MON and persisted at higher BMI.