Some studies indicate that atopy is less important as a predictor of severe asthma. But, remission is more uncommon in allergic asthma than in non-allergic asthma. The purpose of this study was to investigate the prognosis of asthma and risk factors for asthma onset, especially sensitization of specific allergens. A cohort of three age groups responded to a respiratory questionnaire in 1990 and 2003. At baseline, 2060 subjects who reported respiratory symptoms and 482 controls were investigated with interviews, spirometry and skin prick test. A total of 721 asthmatics and 976 subjects without respiratory disease were clinically verified. At follow-up in 2003, 340 subjects with persistent asthma and 186 subjects with asthma remission were identified while 76 subjects reported new asthma onset. Sensitisation to pets and a high symptom score were significant determinants of persistent asthma (ORs 3.23 [95% CI 1.9-5.67], and 5.76 [2.35-13.34] respectively), and onset of asthma, (ORs 2.65 [1.13-4.86.0], and 1.7 [1.21-2.35], respectively). A high self-reported responsiveness to airway irritants (OR 1.6 [1.12-2.2]), and more asthma medications (OR 2.0 [1.3-2.9]) were additional risk factors for persistent asthma at the follow-up. Belonging to the older age group decreased the risk of having persistent asthma or asthma onset. In conclusion, the findings of this study show that asthmatics sensitized to pets have a more severe prognosis than asthmatics not sensitized to pets. Sensitization to pets was also a strong predictor for onset of asthma. Our study indicates that special care should be given to asthmatics who report having problems with a high number of airway irritants as such patients are more likely to suffer from persistent problems.