The effects of immunization with killed influenza virus vaccine were assessed by comparison with placebo in a double-blind study of 318 adult patients with chronic asthma. The patients were randomly allocated to active vaccine and placebo. No difference was observed in peak expiratory flow rate or in clinical symptoms of bronchial obstruction between the groups receiving active vaccine and placebo during the first week after immunization. The data were analyzed separately for age, sex, duration of the disease, hypersensitivity to aspirin (acetylsalicylic acid), atopic status, patients with a history of attacks of asthma induced by viral infections, patients with a diurnal variation of baseline peak expiratory flow of 20 percent or more, and patients receiving continuous oral steroid medication, but none of these factors seemed to predict any short-term adverse effects of vaccination. Follow-up for eight months after the vaccination revealed no differences in asthmatic symptoms between the patients treated with active vaccine and those receiving placebo. The antiviral antibody response to vaccination was normal. The possible protection provided by the vaccination against exacerbation of asthma induced by influenza could not be evaluated, since the influenza epidemic expected during the season failed to occur in Finland. It is concluded that immunization with killed influenza vaccine is safe and is not associated with any significant side effects in adult patients with chronic asthma.