Ocular infection with Chlamydia trachomatis leading to trachoma is the commonest cause of treatable blindness worldwide. Although effectively controlled by improved sanitation, a chlamydial vaccine may prove a more cost-effective method of protection against this disease in the medium term. The development of vaccines against Chlamydia trachomatis has been hampered by a lack of understanding of the immune responses leading to protection or pathology, particularly in humans. Although a strong cell-mediated immune response (T helper lymphocyte type 1) is almost certainly necessary to resolve intracellular chlamydial infection, a humoral (T helper lymphocyte type 2) response may protect against colonization and limit immunopathological events. Recent studies addressing this potential conflict are reviewed.