Chlamydia is the most common sexually transmitted bacterial infection worldwide. National screening programmes and antibiotic treatment have failed to decrease incidence, and to date no vaccines against genital chlamydia have been tested in clinical trials. In this trial participants received three intramuscular injections of 85 microgram vaccine (with adjuvant) (n=30) or placebo (n=5) to the deltoid region of the arm at 0, 1, and 4 months, followed by two intranasal administrations of 30 microgram unadjuvanted vaccine or placebo (one in each nostril) at months 4·5 and 5·0. The results obtained should provide enough evidence of potential to provide the impetus for phase 2 trials. The practical difficulties of testing for efficacy in Phase 2 trials are discussed the associated commentary.