Abstract Fifty patients with chronic obstructive lung disease were randomly allocated to three groups, to assess whether an oral vaccine containing non-typable Haemophilus influenzae protected against acute bronchitis. The double-blind prospective study over a three month winter period included two placebo groups and one test group. Oral immunisation with H influenzae induced a tenfold reduction in the incidence of infection (p<0·001). During the subsequent winter, without further immunisation, protection by the vaccine was no longer statistically significant. There was no clear correlation between clinical protection and either carriage of H influenzaeor the level of antibody to H influenzae antigen in saliva.