The very great increase in the tourist industry during the past 20 years is described. This has occurred at a time when there has been an unprecedented rise in the incidence of sexually transmitted diseases. Evidence is presented that there is a relationship between the number of tourists and other travellers and the prevalence of sexually transmitted diseases. Case histories from three different categories of tourist are described and the impact of tourism on the spread of disease is stressed. It is suggested that health authorities throughout the world should give further consideration to plans to meet the challenge of the mobility explosion.