The value of antibiotics was assessed in a randomised, double-blind study of amoxycillin and placebo in sixty adults admitted to hospital with acute exacerbations of asthma. 37 exacerbations were treated with amoxycillin and 34 were treated with placebo. Response to treatment was closely monitored but no significant difference in improvement was demonstrated between groups for length of hospital stay, time taken for 50% improvement in symptoms, patient's self assessment and respiratory function, and symptoms and respiratory function at time of discharge from hospital. Antibiotics should not be given routinely to patients admitted to hospital with acute exacerbations of asthma.