When the body is invaded by pathogens, each individual reacts with a battery of non-specific and specific immune defence mechanisms. Sex hormones play an important part in the host's resistance to sexually transmitted infections. This is shown by sex differences,1 variations in detection of infection according to phase of the menstrual cycle,2 increased susceptibility during pregnancy,3 and evidence that use of hormonal contraception predisposes to infection. 4 5 The mechanisms by which specific sex hormones modulate the immune system have been reviewed.6 This is a complex subject, and it is unfortunate that, in spite of its potential importance, no clear message about sex hormones has emerged or been harnessed to the development of strategies for controlling infection. This paper revisits some of the evidence that hormonal status influences susceptibility to sexually transmitted infections, especially in adolescents, and considers the practical implications of this evidence.