Summary Background Asthma patients exhibit an increased rate of loss of lung function. Determinants to such decline are largely unknown and the modifying effect of steroid therapy is disputed. This cross-sectional study aimed to elucidate factors contributing to such decline and the possible modifying effect of steroid treatment. Methods We analyzed determinants of lung function and airway hyperresponsiveness (AHR) in a Scandinavian study of 2390 subjects from 550 families. Families were selected for the presence of two or more asthmatic children as part of a genetic study, Scandinavian Asthma Genetic Study (SAGA). Results The primary analysis studied the association between the lung function and delay of inhaled corticosteroids (ICS) after asthma diagnosis among asthmatic children and young adults with a history of regular ICS treatment ( N=919). FEV 1 percent predicted (FEV 1% pred) was 0.25% lower per year of delay from diagnosis until treatment ( p=0.039). This association was significantly greater in allergy skin prick test negative children. There was no significant influence of gender, age at asthma onset, or smoking. In the secondary analysis of the whole population of 2390 asthmatics and non-asthmatics, FEV 1% pred was inversely related to having asthmatic siblings (−7.9%; p<0.0001), asthma diagnosis (−2.7%; p=0.0007), smoking (−3.5%; p=0.0027), and positive allergy skin prick test (−0.47% per test; p=0.012), while positively related to being of female gender (1.8%; p=0.0029). Risk of AHR was higher by having asthmatic siblings (OR 2.7; p<0.0001), being of female gender (OR 2.0; p<0.0001), and having asthma (OR 2.0; p<0.0001). Conclusions These data suggest that lung function is lower in asthmatics with delayed introduction of ICS therapy, smoking, and positive allergy skin prick test. Lung function is lower and AHR higher in female asthmatics and subjects with asthmatic siblings or established asthma.