To explore whether asthma and obesity share overlapping pathogenic features, we examined the impact of each alone, and in combination, on multiple aspects of lung function. We reasoned that if they influenced the lungs through similar mechanisms, the individual physiologic manifestations in the co-morbid state should interact in a complex fashion. If not, then the abnormalities should simply add. We measured specific conductance, spirometry, lung volumes and airway responsiveness to adrenergic and cholinergic agonists in 52 normals, 53 asthmatics, 52 obese, and 53 with both problems using standard techniques. Six-minute walks were performed in subsets from each group. Asthma significantly lowered specific conductance and the spirometric variables, while increasing airway reactivity and residual volume. Obesity also reduced the spirometric variables, as well as total lung capacity and functional residual capacity. Residual volume, specific conductance and airway responsivity were unaltered. With co-morbidity, the disease specific derangements added algebraically. Features that existed in isolation appeared unchanged in the combination while shared ones either added or subtracted depending upon the individual directional changes. Synergistic interactions were not observed. Body mass index weakly correlated with spirometry and lung volumes in asthma, but not with specific conductance or bronchial reactivity. Exercise performance did not aid in differentiation. Our findings indicate asthma and obesity appear to influence the respiratory system through different processes.