Influenza epidemics of variable extent and severity occur every winter and are frequently associated with exacerbations of asthma. Accordingly, annual vaccination against influenza is recommended for patients with asthma. However, there are very limited data concerning its protective effect in this group of patients. The aim of this study was to assess the effect of influenza vaccination on the frequency of upper respiratory tract infections and also asthma-related outcomes such as exacerbation rates, hospital admissions, and rescue courses of oral corticosteroids in patients with stable asthma. Between September 15 and November 7, 2001, a total of 128 patients with asthma were randomly assigned to receive (n = 86) and not to receive vaccine (n = 42). The primary outcome measures were frequency of upper respiratory tract infections and exacerbations of asthma during the winter following vaccination. Study subjects were asked to record the presence and duration of symptoms suggestive of an upper respiratory tract infection and call their physician in the presence of conditions suggestive of an exacerbation until March 2002. Among the vaccinated group, 48% of the patients reported that they had no upper respiratory tract infection during the winter following injection, whereas 57% of nonvaccinated participants were upper respiratory symptom free during the same period (p > 0.05). The frequency of upper respiratory tract infection was also not different between the two groups in all severity forms of asthma (p > 0.05). There was no significant difference in the frequency of exacerbations of asthma between the two groups during the study period (p > 0.05). None of the vaccinated group was hospitalized due to an asthma attack; however, two patients (4.8%) in the nonvaccinated group had to be hospitalized following an exacerbation (p > 0.05). In summary, our findings do not support the protective effect of influenza vaccination for patients with asthma. However, no firm conclusions on this effect of the vaccine can be made without the data on the rate of influenza epidemic in that season and without the knowledge of the cause of upper respiratory tract infections in those patients. Therefore, we believe randomized, double-blind, placebo-controlled studies, including larger subgroups of severe asthmatics, are needed to evaluate the protective effect of influenza vaccination in asthma.