Asthma is associated with several comorbidities but the magnitude of the association has not been clearly defined. We aimed to examine the relationship between asthma and comorbidities using information obtained from the Health Search Database (HSD) owned by the Italian College of General Practitioners (Società Italiana Medici Generici, Florence, Italy). We conducted a population-based retrospective study using information obtained from the HSD. The software system used codes of all the diagnostic records using the 9th revision of the International Classification of Diseases. Asthma appeared to be weakly associated with cardiovascular and hypertensive diseases. Intriguingly, the odds ratio of acute or old myocardial infarction was 0.84 (95% CI 0.77-0.91). Asthma was also weakly associated with depression, diabetes mellitus, dyslipidaemia, osteoporosis and rhinosinusitis. In contrast, it was strongly associated with gastro-oesophageal reflux disease (GORD) and, particularly, allergic rhinitis. Age did not influence the association of asthma with comorbidities whereas sex had a different impact according to the specific comorbidity. Our results indicate that asthma is weakly associated with several comorbidities, whereas its association with allergic rhinitis or GORD is stronger.