To update the WHO global and regional estimates of the prevalence and incidence of syphilis, gonorrhoea, chlamydia, and trichomoniasis. Prevalence estimates for syphilis, gonorrhoea, chlamydia, and trichomoniasis were generated for each of the nine UN regions for males and females between the ages of 15 and 49 in 1995 based on an extensive review of the published and unpublished medical literature since 1985. Incidence estimates were based on the prevalence figures and adjusted to take into account the estimated average duration of infection for each disease in a particular region. The latter was assumed to depend upon a number of factors including the duration of infection in the absence of treatment, the proportion of individuals who develop symptoms, the proportion of individuals treated, and the appropriateness of treatment. In 1995 there were over 333 million cases of the four major curable STDs in adults between the ages of 15 and 49--12 million cases of syphilis, 62 million cases of gonorrhoea, 89 million cases of chlamydia, and 170 million cases of trichomoniasis. Geographically, the vast majority of these cases were in the developing world reflecting the global population distribution. STDs are among the most common causes of illness in the world. Estimates of the global prevalence and incidence of these infections are limited by quantity and quality of data available from the different regions of the world. Improving global STD estimates will require more well designed epidemiological studies on the prevalence and duration of infection.